During the Second World War, amateur telescope makers in the United States produced roof prisms for military use. Albert Ingalls, then an editor at Scientific American magazine, was the major organizer of the group. These are very difficult optics to make using amateur equipment, and the story of how this group, perhaps 40 people, were recruited, trained, and supported, is a fascinating sidelight of history. Records and information are not easy to obtain, and there does not seem to be anything on the WWW on this topic. In an attempt to attract more attention to these workers, several articles from 1940s issues of Scientific American were scanned, rewritten, and posted at this site. Hopefully, this will bring in enough input to allow a proper history to be written. ============================ Albert G. Ingalls. A Hobby Goes to War: Scientific American's Amateur Telescope Makers Find Their Peacetime Optical Skills are a Wartime Asset. Scientific American, May 1943, 202-205. "Desperately - that's how badly they are needed." It was a year and a half ago and the Army procurement official spoke in a suddenly lowered voice. The thing so desperately needed, a need now no longer desperate, was something you may never have heard of - roof prisms. Essential part of telescope sights for field guns, anti-aircraft guns, tanks. Little pieces of crystal-clear optical glass the size of your thumb, sparkling like cut gems, two of their facets sloped like the sides of a roof. Worked to a geometrical nicety within two millionths of a circle in exactness of angle between the facets. Among those supreme tasks which are tackled only with deference by precision optical workers, almost none calls for greater skill of hand, and eye, and brain than making roof prisms. Commoner kinds of prisms there are, but roof prisms are precision royalty. A good roof prism maker is the equal in military value of a whole company of soldiers. A year and a half ago there were not in these United States as many as a score of precision opticians who could make these prisms. In peacetime one or two men could easily supply the normal needs of all the military services, and roof prisms had not been used for other purposes. Suddenly a nation's new armies needed thousands of them. Without its roof prism a gunsight, and therefore a gun, could not function. The Army procurement official's depiction of a desperate situation, therefore, was no empty piece of dramatics. Some of the few who could make roof prisms had long been employed at Frankford Arsenal. Consequently, to that institution were sent the ablest artisans, picked men, from the precision optical industries - those that in peacetime make scientific instruments - to work by the side of its experts and take home their secrets to impart to new workers. If this skill-multiplying process were as simple as it sounds, there would have been roof prisms, easily and early, but it wasn't simple at all. Suppose that Stradivarius had been called on to increase his production of violins a hundred fold within months. It was more like that. Men don't learn such minutely exacting arts in a day - not even professionals who have done other kinds of precision optical work for years. There was, however, another group of men in this country who likewise for years had been doing precision optical work, though as a hobby - the amateur telescope makers. Among these 20,000 followers of a scientific sport there are many who have developed skills in making lenses and concave mirrors for telescopes demonstrably equal to those of professionals. Could some of these eager, patriotic amateurs, working at home in their cellar and attic shops, help relieve the Army's roof prism need? Preceptor of the American amateur telescope making hobbyists is Russell W. Porter. Early in the century, during a dozen years as astronomer and surveyor with several arctic expeditions, Porter dreamed of owning his own astronomical telescope. Cured at last of his "arctic fever" he returned to New England and there in his cellar shop he made a telescope of the reflecting type, a type which he called "the poor man's telescope," because its simple materials cost less than those for the more familiar kind having a large lens. Later he taught the same art as a hobby to a group of average members of his home community. These amateur telescope makers organized a club, and on the top of a neighboring mountain they built a clubhouse- observatory, naming it "Stellar Fane"—shrine to the stars. Today Stellar Fane, or Stellafane, is the mecca of American amateur telescope makers, and the club that Porter started is the prototype of of similar clubs all over the nation. Porter, after making many more telescopes, was called to California to help design the great 200-inch reflector which will remain unfinished until after the war. In 1925 I visited Porter and Stellafane, and there I became inoculated with the virus of the telescope making itch and went homc to make my first telescope. Here; as it proved, was a real hobby, a tough one. By hand, and using loose abrasive grains with plenty of elbow grease, you ground a disk of thick plate glass or pyrex the size of a butterplate to a concave curve. Next you polished it with optician's rouge on a layer, or lap, of pitch. Then, by rubbing it locally, you shaped its shallow depression everywhere within two millionths of an inch of a parabolic curve. To accomplish this was a fight, but a fight that was fun. Finally, you silvered your disk and set it in a tube on a home-made mechanism and found the statement true that with such a telescope you could do serious astronomical work. It would magnify 100 times and reveal the polar caps of Mars, four of the satellites of Jupiter, the rings of Saturn, countless celestial suns and universes. After Porter had helped me fight out the final curve of my first telescope mirror, we sat in a New York chop house from six till twelve one night, sketching telescopes on a tablecloth and hatching a conspiracy to start germ warfare. With his greater experience in telescope making and my access to the pages of Scientific American, we would expose men of amateur scientific turn of mind all over the nation to the contagious bug of the telescope disease. Porter prepared articles which I expanded into a book. It proved that the bug would take, and would even spread without promotion. As soon as a man made a telescope and his townsmen saw it over the fence, they came down with the same itch and set eagerly to work in their garage, attic, or cellar shops. Some would finish their first telescope six inches in diameter and hanker for one eight, finish that and begin one twelve. Club groups made large telescopes for community use. An Indianapolis group made a three-ton reflector three feet in diameter. Telescope making as a hobby became epidemic and still is. Wives saw their husbands dive into cellar shops after supper or without it, and come up only after midnight or when hungry. The long-suffering, deserted ladies called themselves "glass widows." One, in revenge, bolted her spouse downstairs in his shop all night. Another, in California, sued for divorce and won it. Scattered over the nation are thousands of these enthusiasts. By mail they scrape mutual acquaintance and become fast friends who have never seen one another, never heard one another speak. Throughout the 18 years of this hobby's career Russell Porter has been its father - my part more motherly, having to do with the homelier conduct of the central clearing house for news and ideas. While not all amateurs' telescopes are quite Grade A, many are easily as precisely made as those that professionals produce, and some are even better. For this apparent anomaly there is a reason that seems to make sense. The amateur can work on and on till every last detail is perfected, even if it costs him a year of spare time, but the professional must count his time or else lose money. The word "amateur" means one who does something for the love of it and not for gain. But, because some who make no gain are also inexpert, the ill-starred word has also acquired the meaning of bungler and dilettante. During more than a century of the telescope making art the ablest and most famous professionals have been men who began as amateurs, and then actually beat the professionals at their own game. For this there are parallels in other hobbies and in sports. Such men never fall out of love with their work. Though professionals, they remain amateurs - devoted lovers - in outlook. Many of the amateurs not only refused to stop work after making a single telescope, but at one kind of telescope, and even at telescopes. With a loose foot and a roving eye they explored the diverse side-roads of optics. They made unusual and uncommon telescopes and domed observatory buildings in which to house them; sun clocks, camera obscuras, spectroscopes. They also made optical test planes, called flats - glass planes that deviate from absolute geometrical flatness not more than one millionth of an inch anywhere over a surface the size of a dinnerplate. Flats have many uses in precision optics and as final controls of all measurements, in industry. The amateurs not only made these things, but often the same men made the tools with which to make them. As they worked, they also pored over profound books, largely mathematical, on optical theory and design. In long periods of years - five years, ten years - a hobbyist often learned more about precision optics than he sensed. This background of experience was destined to prove an asset when the United States Army needed roof prisms, and that is the other part of this story. In 1937, when this nation was seen to be headed for war, a hint was thrown out among amateur telescope makers who read Scientific American that, if war came, the widespread reservoir of precision skill among their personnel might do its bit to help win it. One far-seeing amateur who thought often about this was Alex MacTavish. He foresaw that war would call for roof prisms for gunsights and he correctly reasoned that, because this particular variety of prisms was the hardest thing in optics, they would prove to be needed the moist. This acted as a challenge to his nature and, as he went about his vocation, his mind dwelt often on roof prisms. Strongly encouraged and in many ways aided in this by Porter, in 1940 MacTavish had already quit his business, built his shop, designed and constructed his machines and testing equipment, and ferreted out a procedure for making roof prisms. Thus he had gained a head start over the other amateur telescope makers, and when these later tackled roof prisms he was best qualified to be the technical advisor of the group of those who today are successfully turning out a sizable share of the roof prisms needed by the Army. To Frankford Arsenal, then, a year and a half ago, went Porter, MacTavish and I - three musketeers in quest of an arrangement by which other amateurs could try making roof prisms and, if they succeeded, produce them regularly in their spare hours as a contribution to victory. Would the quest prove quixotic? Would the Musketeers be laughed at - amateurs, of all people, aspiring to make roof prisms, of all things! Why, being amateurs, didn't they tackle something relatively simple - perhaps the easy Porro prisms used in binoculars. Some people, in fact, said they were just a bit crazy, but sometimes it's an advantage to be crazy. If any official at Frankford Arsenal thought such things, his exterior remained impeccable. How can an official judge in a jiffy whether he is listening to wild men or passing up a good bet? And there was also that desperate need, so.... Had the amateurs been making roof prisms? Yes and no - MacTavish had, but the others hadn't tried it yet. What, then made them think that the amateur telescope makers could manufacture these especially difficult prisms? The fact that they had made other difficult precision optical things. MacTavish, for example, before tackling roof prisms, had made small lenses, three telescopes, three flats. Had they come to ask for a big contract? No. Only for the simple assurance that if the amateurs tooled up and made sample roof prisms, these samples would be officially tested and rated for quality; and that small trial contracts would then be given those who showed promise. A small amount of optical glass with which to make samples was also asked for. Frankford Arsenal bet 24 two-ounce pieces of optical glass and later, when the amateurs still seemed serious, 100 more pieces. Optical glass then was scarce - it always is a treasure. In fact, this exquisite substance truly merits a name wholly distinct from the commoner word, glass; just as roof prisms deserve a name distinct from the word prism, and the precision opticians who work in millionths of an inch on optical glass should have their own separate designation. Here our language lacks three words. The glass was to be distributed by me, two blanks to a man, to those of the advanced amateur telescope makers, where ever they lived who were known to have outstanding skill. On behalf of Scientific American I agreed to co-ordinate the group of about 80 candidates scattered over the nation, transmit their sample prisms to the Arsenal for test, and sift out the men who made good, turning them over to the Arsenal as contracting producers. This made of me a sort of old mother hen for the venture. Home went three musketeers, well enough satisfied. You couldn't expect an arsenal to go way overboard at once on amateur stock. It now was up to the men. A "Roof Prism Program" was organized, but, since amateurs are a clubby, unconventional lot with no leanings toward the pretentious, the movement was called simply "The Gang," which sounded less like Washington. The participants were mature, middle-aged men with established vocations. A sampling of the Gang runs like this: a metallurgist, a biologist, a steel worker, a cabinet maker, two physicists, two chemists, a candy manufacturer and a gravestone manufacturer (who work as partners), several engineers, an accountant, a physician, dentist, decorator, geologist.... A group in one community comprised a microscopist, teletype engineer, telephone engineer, paleontologist, and a herpetologist (snakes)—all working in spare hours. Practical instructions for making roof prisms were the most immediate need. Alas, none had ever been published. Only a writer who had made roof prisms with his own hands, but no mere sideline observer, could properly qualify to write such instruction, and none of the few who had done so had ever gone literary. Among professionals there also was an old tradition that the subtleties of the higher precision optical skills were too elusive to capture and put into writing. This rationalization had never been accepted by the amateur telescope makers, because daily for 15 years they had been doing exactly that. True, it required painstaking description - precision language - to explain precision optics. Books, articles and long letters, all written by amateurs for other amateurs, had been almost the sole transmission lines for the amateur's working techniques of telescope making. Instructions for making roof prisms, so far as the instructors' incomplete knowledge permitted, were therefore prepared. Porter, in California, wrote a booklet in which he embodied the data which he had garnered about testing prisms - for it is essential that the angle between the rooflike facets be square within two seconds of arc, or 1/1800 of a single degree. How infinitesimal such an angle is would be shown if you could see a man at 30 miles. Light rays coming from his left and right shoulders to your eye would then make that angle. Bringing the roof angle by hand retouching to this high precision - far too fine to be measured with any kind of mechanical gage, and measured instead with long pointers of light that amplify the errors - and at the same time keeping the determining surfaces flat - is the chief reason why roof prisms are far more difficult to make than ordinary prisms. Because nobody had ever taken the trouble to write a treatise on the roof prism art, MacTavish had been forced to invent, all over again, what may have been known 50 years before and largely lost. All that he had thus painfully learned he at once put into writing and gave to the others freely, through mimeos issued with chipped-in funds. This patriotic gift enabled them to start close to the stage which he had currently reached. Herschel Ice, an amateur who had worked professionally for six months at Frankford Arsenal, remained faithful to the spirit of his antecedents, by writing out for the Gang all he could glean there about prism work. A mimeographed, monthly Gang newsletter also was begun. Its purpose was to remedy an existing fault - that these men could not, at this uncertain stage, quit their regular vocations to come together at any central point to work, as otherwise would have seemed logical. The newstetters brought the men together once a month in a sort of town meeting. Through them, every scrap of knowledge about the technique of roof prism making which any one Gang member discovered and sent in could be passed to all the others. Between the issues of these monthly Gang gossip sheets there came to my desk a daily blizzard of letters - trouble letters, rejoicing letters from men who had found new ways to do old things; letters from those who couldn't buy this or that tool because they lacked priorities; letters, letters, letters. These many, many missives were answered - let bureaucrats and red tape winders brace themselves - with entire informality on common postalcards, but on the day of receipt and generally in the same hour. The same letters were then bundled up each day and sent around the nation-wide triangle - MacTavish, Porter, and back - so that the other two musketeers could keep in close touch with the activities and contribute their helpful comments to each inquirer's problem. "If all these letters could be put end to end," Porter's secretary grinned, "they would paper a path to the Moon." Tooling up for roof prism making cost only one or two hundred dollars because the men already owned part of the equipment. They had used it in telescope making. All were natural mechanics and could make the remainder. For example, professionals usually purchase the grinding spindles they use, but amateurs could roll their own from junk. The spindle made from junk was as good as the one that looked like a standard catalog machine, except that it lacked paint and polish. The required optical flats, or test plates for glass, were no great problem for men who had years before learned to make them accurately to a millionth of an inch or better. Did the complication of obtaining a priority seem to block the hope of acquiring a necessary high-precision square or gage? Then make it yourself. Did James Fogarty have to buy a saw for sawing glass when he could crush a black diamond and arm the edge of a brass disk with its powder? Did one man in the Gang feel uncertain about the metal he used for a fine tool? Very well, the Gang included a metallurgist who knew the answer. Did the metallurgist want to make his own rouge, but feel uncertain about chemical details ? The Gang contained chemists and one man's letter could be passed on to another. After the start there were a few weeks of suspense. One morning came a big box. Lovingly protected at the center of about a cubic yard of packing was a sparkling, transparent roof prism of water-clear optical glass. They're common now, but the first one seemed like a maharaja's gem. In a mountain village, Pavel Uvaroff, to whose door people had previously beaten a path even from China for special jobs on telescope mirrors, had labored a hundred aching hours on this one prism; though today, as a veteran of a year, he'd do it in an hour. Thc elaborate packing bespoke the pain it had cost him. Uvaroff's prism - the first to come in - was forwarded to Frankford Arsenal for test and the report showed that the roof angle - most critical of numerous details that had to be right, or else - exceeded the desired right-angle by several times the permitted tolerance. Yet for a first shot this was excellent shooting, easily on the target and it didn't take Uvaroff long to begin hitting the bullseye. Within the next fortnight three more men had sent in their long- labored-over first samples. Impatient, I put these in my pocket and went to the Arsenal to see them tested. I was apprehensive, since first impressions count and a bad showing now might prejudice the amateur's future. The foreman, 30 years at Frankford Arsenal and a master optician, tested. Would such a man be scornful of the amateur? His sunny demeanor said no, as the prisms were placed on the test stand. He squinted through the eyepiece of the apparatus. First man's prism: Angle error five times the tolerance. That wasn't too reassuring. The second man's prism was nicked and scratched like the chin of a New York commuter after his hurried morning shave; perhaps, therefore, it would prove to be good in its more essential characteristics - like a plain wife with a wonderful disposition. Alas, the angle was a whole thirtieth of a degree from square - 60 times the tolerance - and the facets were far from optically flat. Such a prism would give only blurry definition if used in a gunsight. Amateur stock was falling fast. Third man's prism - Jim Fogarty's. Test. Suspense. Foreman: "Good prism! Definition fine - very good. I know this man knows what he's doing - he won't have any trouble reducing his angle to two seconds." The Arsenal gave Fogarty a trial order for 50 roof prisms and when he completed it, 47 of them proved to be acceptable. Fogarty's average on the hundreds of prisms he now makes runs much higher than this - higher, in fact, than that of many professionals, as does the average of others of the amateurs. This gave the program three men who could make roof prisms. Robert Gray sent in samples. Since he had previously made several telescopes, including a trim one-ton reflector 20 inches in diameter, his work on prisms obviously should shine from the start. At first it proved to be only fair. Not much later, on his trial order, he made a 100 percent score of acceptance. This caused an official to comment that Gray had been thrown clear out of the amateurs and into the professionals, but he refused the intended honor. An amateur is an amateur forever, he commented. Hobbies engender strong loyalties. Today, Gray turns out roof prisms by the peck. By summer several more men had qualified by showing promise in their sample prisms. An official remarked that the amateur were beginning to produce results that none had expected. Oho! Results that none had expected! So now the cat was out of the bag. Yet, in all fairness, no amateur had ever expected anyone to expect, since no amateur had himself felt sure, until the test had been made, that amateurs could make roof prisms. So the Gang members only chuckled and worked on. For Frankford Arsenal had been fair and square, its personnel invariably courteous. It is, of course, a fact that judicious co-operation with civilians is no more than the plain duty of a government arsenal. That is, in time of peace to keep alive a spark of certain rare skills, and in time of war to disseminate those skills among the people whose taxes support the public institution. Yet this overlooks traits in human nature which could easily have supervened had these officials been small, but which did not because they were large. How are roof prisms made? Are they molded, or sawed, or blown, or ground, or what? From thick slabs of optical glass they are sawed roughly to shape with circular saws whose microscopic teeth are fine abrasive particles. Sometimes, instead, they are molded to a rough outline. Next, numbers of the rough blanks are cemented to iron plates and together ground more closely to the desired size. Groups of these are then cemented into V-shaped grooves in prepared plates of steel and ground still more closely, more delicately, to size. The prisms are then polished in groups with pitch laps, using optical rouge, and their facets made optically flat. Each step is carefully gaged to bring the prisms ever closer to the ultimate shape and size. On each prism during these processes at least 50 accurate measurements must be made. The final stage, or "hand correction," is done by rubbing off glass freehand on a rouged pitch pad, at less than a millionth of an inch per rub, with fingers so practised that the optical flatness already gained is not lost while the angle is being corrected. This calls for supreme skill. To the Gang came jolts as well as joys. Men became over-confident and received sudden heavy rejections. Men who had started off with a bang slumped. A number of new men failed from the first. But Daniel Widdicomb sent four samples, all of which proved to be acceptable, and he, today, is a producer of roof prisms and has a shop in his cellar, employing two assistants. No imposing factory facade is necessary in work like this - only simplicity and the essential skills. You can't bluff an inspector's testing apparatus with an embossed letterhead. Some members of the Gang combined. Thus, Strong and seven nearby amateurs worked together in one little shop, so packed with tools and work that a man would pop out the walls if he took a deep breath. Five men, led by George Ellis, co-operated in one shop. Dakin combined with Sloane. Uvaroff, whose superb workmanship soon brought him a larger order for 300 prisms, worked as a partner of MacTavish, whose second order was for 1700. Together these two work in a temporary barn-like building near MacTavish's residence, their combined orders for several thousand roof prisms now nearly complete, and with an exceptionally high acceptance of better than 98 percent. Just exactly why were these amateurs making roof prisms? Patly put was Fogarty's answer to this question: Patriotism, pride, profit - and profit last. "My main ambition," wrote Hartshorn, "is to get something made with my own hands into U. S. fire control equipment and doing things to Berlin and Tokyo. If I can accomplish this, I'll feel well repaid for my work, and to Hell with the money." I could quote nearly every participant in this romantic quest in a similar vein. A second motive was pride - the very challenge of the difficult roof prism. Over-compensation for being amateurs? Quite possibly. Naturally this hit the amateur harder than the professional, since many of the latter had never tackled roof prisms, and some who did failed dismally. In the third motive - profit - there is no moral crime, but one member of the Gang, who can make roof prisms and makes hundreds, succeeded in getting himself suspected of some hidden mysterious motive when he tried to donate his profit to the war effort. The officials are still in a swoon. Throughout the whole period the members looked mainly and often to MacTavish for sound advice about the working technique. This dynamic (red- headed) overcomer of obstacles has the most roof prisms to his score. If discouraged, he smiles. If worse discouraged, he grins - and works. Things gravitate to such people. Late last summer ten of the Gang took a day off and met in Frankford Arsenal, most of them seeing one another's faces for the first time, though all were old friends by mail. By then the period of tiny trial orders was past and Frankford knew that amateurs could make roof prisms. Half a ton of optical glass was being shipped to eight of the ablest men. These soon made it into roof prisms and asked for more Did the amateurs find roof prisms more difficult than telescope making? Much more so. From first to last there were troubles, and heartaches, and headaches. There were cycles of elation and depression which chased one another down the keyboard like fugues, and they still chase. About them a whole book could be written and that book would not be fit to print. At least one fat chapter would be about Washington-inspired red tape. But who among readers cares about tales of woe? Such troubles, when philosophically looked at, are only a part of war, and war is Hell. Thus the Musketeers' original purpose - to sift a carefully selected group of the abler amateur telescope makers, in the hope of finally sifting down to a smaller number who could and would make prisms toward Victory, and who together would contribute a volume of roof prisms comparable with the production of one of the professional plants - had justified itself. It is suspected that Scientific American's Amateur Roof Prism Program and its Gang must have caused Frankford Arsenal almost as much trouble as an equal number of large manufacturers - or as a hen with a hatching of ducks. That the Arsenal, after all its patient dealings with the amateurs, still stands solidly on the banks of the Delaware, and still can function, clearly establishes its toughness and indestructability. From its officials there came recently to Scientific American the following communication: "In the Fall of 1941 this Arsenal became interested in a group of amateur telescope makers sponsored by your publication. Since this time, a good many of these amateurs have developed into qualified optical workers. They have turned in roof angle prisms which meet the very high quality of performance required by military instruments. Several of them have gone into this work on a full time basis, and have established shops capable of turning out precision optical elements in commercial quantities. They are able to compete with larger organizations both on a quality and cost basis. The development of this whole program is much more a result of the work of your publication than of this Arsenal. The instruction of the amateurs was almost wholly accomplished through the medium of the mimeographed booklets prepared and issued by members of your staff. You have at no time received any financial gain from this work, but have offered your services simply as a contribution to the war effort. This Arsenal, therefore, takes this opportunity to thank you for the interest you have shown, and to express its hope that you will continue this work." The work does continue and runs now on its own momentum, each member of the Gang an independent producer and no longer in need of assistance. The wartime fulfillment of a peacetime hobby! ============================ Roof Prisms (an Addendum to the Article "a Hobby Goes to War", page 202f.). by Albert G. Ingalls. Telescoptics, Scientific American, May, 1943. THE FOLLOWING notes in this month's Telescoptics department are an addendum to the general article on page 202, which should be read first. They are aimed at the reader who has made telescopes, but not roof prisms. THE FAMILIAR right angle prism turns I rays through an angle of 90 , inverts the image, but does not reverse it. The elbow, telescopes of military gunsights on the other hand, require a prism that will similarly turn rays through an angle of 90 , similarly invert, but will also reverse the image. In actual application of course, such a prism is called on to normalize the already inverted and reversed image produced by the gunsight's telescope objective. The common right angle prism can be modified so that it will do all these things. Russell W. Porter briefly explains this modification thus: "Let us start with the familiar right angle prism A, Figure I, top, indicated with light lines. We grind away two sides of its hypothenuse face until only the line BC is left. This gives us two faces, D and E, which we shall call the roof. The sides of the roof must make an angle to each other of 90 . "Now consider a beam of light entering the prism from F and emerging at G, at right angles to the immergent beam. If two arrows are used to represent the incoming object, it will be seen that, on emerging, the image has been reversed, as the arrows show. "By tracing the arrow points and tails through the glass, shown by dotted lines, their rays are seen to cross each other by internal reflection from the roof faces. The image is also turned through an angle of 90 ." It may prove difficult to visualize this from a drawing, if the reader is not already familiar with roof prisms-it usually is confusing even to those who, for the first time, actually examine a roof prism. At first one looks very complicated. An added reason for this confusion is the fact that in such a prism there apparently are nine facets. Five of these are not, however, true optical facets The two tips of the prism which lie outside the circle of rays that reach the entrant face, as well as the heel part of the right-angle portion which lies outside the telescope's aperture, are ground off to save space in the mounting, and are left fine- ground, as are the two sides. Thus a manufactured roof prism has only four polished, flat, optical faces. These are: the entrant face; the two roof faces which were made over from the right angle prism's hypothenuse side and which are at exact right angles to each other; and the emergent face at right angles to the entrant face. G. B. Amici (1786-1864), an Italian, invented this prism, which is sometimes called the Amici prism. Sometimes, too, it is called other names not fit to print, by those who wrestle with its difficulties. Ordnance Document 1065, prepared under the direction of the Chief of Ordnance by Dr. I. C. Gardner, Chief of the Optical Inspection Section of the National Bureau of Standards, states: "It is one of the most difficult to manufacture because the roof angle cannot differ from 90 by more than a few seconds. Angles having the requisite accuracy cannot be produced directly, but must be carefully approximated by usual manufacturing methods: after which the faces must be carefully polished by a skilled operator and individually tested until the angle is so nearly correct that the image is not doubled. An error of a few seconds is sufficient to make the prism worthless. This tedious method of production limits the output, as few men become sufficiently skilled to do this local retouching." Local retouching and "hand-correction," mentioned in the article for general readers elsewhere in this number, are the same thing. Roof prisms are used in elbow telescopes, and elbow telescopes are used in all anti-aircraft sights, in order that the observer may look horizontally into the eyepiece. In the panoramic field-gun sight a horizontal element with a rotating objective prism of right angle type is added to the top of the elbow telescope, so that the cannoneer, without changing position may sight, first on his "aiming point" (not the actual target but some convenient fixed reference point), then turn off his azimuth, corrections for windage, drift, and so on, and fire the gun. This is the method of indirect laying of guns. That method is predominant because today the gun usually remains concealed and the firing data are given to its crew: by telephone from a distant observer who can see the target. This explains why a straight, direct, telescope, used as a sight, which would seem to be the answer that would do away very simply with the roof prism problem, could seldom be used. Every amateur telescope maker will be curious to gain a rough idea of the high spots of the procedure of making roof prisms. To this end MacTavish has contributed the following, by invitation: "In Figure 1, bottom, A is a side elevation of a roof prism, showing the 90 angle ('end angle') between the entrant (immergent) and emergent faces, also the 45 angle between the roof line and the entrant and emergent ('end') faces. In addition, the roof faces are 60 to the end faces. "In producing the prisms, all angles except the roof angle are controlled in grinding and polishing to 2' of arc. This is assured if there is a light-tight reading under a fine standard square and a 60 gage. The roof angle is then brought to 90 +/- 2" by hand correction on a flat lap of pitch. "The four optical facets must be flat within standard precision tolerances. Pits, scratches, grey surfaces, striae, ream, bubbles, nicks, and small fractures are not tolerated. "Mechanical dimensions are held to close tolerances. "The following roughly outlines the main processes, omitting the finer points, also the troubles-the former would add perhaps 25,000 words to the account, the latter from 50,000 up to, say, 1,000,000. "The work starts with thick slab glass, though pressed forms are also used. "The glass is sawed into rectangles, each large enough to make two prisms when later sawed diagonally. "Groups of these are cemented to flatiron plates and held against wet abrasive grains on round, rotating, flat, iron plates, (Figure 2) first on one side, then the other, till they mike correct to the prism thickness dimension specified. "Several of these squares are cemented together in stacked-up form, and one side 13 and one end of this block are ground square to the sides and to each other. "The same cemented group is cemented to an optically flat iron plate with hot wax and the other side and end are ground parallel to the first side and end, respectively, in several operations. "The pieces are removed, uncemented, and each square is cut diagonally with a r. diamond saw. "The sharp tips are then ground off and left fine ground permanently. "The 90 angles are corrected by hand with fine emery; otherwise the roof-face-to-end-face angle will depart grossly from 60 , and the roof- line-to-end-face angle will depart grossly from 45 . Other sources of error at this point must be skipped. "Next, the two roof faces are ground on. The prisms are inserted in accurate metal V-blocks (Figure 3), with V's to hold them laterally, also notched metal inserts to hold them longitudinally (these are shown disassembled in Figure 2), and the roof faces are ground on, first one face of all, then the other face of all. The roof faces are miked uniform all around through the V-block, so that the roof ridges will be in the exact center of the prisms. "After the roof faces are ground on, the 60 angles are corrected by hand retouching against an iron plate with fine abrasive-a very ticklish, fatiguing task. "Next, the sharp edges of the prism are beveled. "The prisms are again cemented, bottoms up, into another V-block without inserts and the small (non-optical) bottom faces are ground off. "Preparation for polishing is begun. Face by face the prisms, somewhat separated, are cemented to an optically flat iron plate with melted paraffines or cold oil. A ring of sheet metal is placed around this plate and special plaster is poured around the prisms. This forms a matrix, or round block. The ring is removed, the bottom iron plate is warmed (if paraffine is used), and the block slid off. The plaster is cut back 1/16", so that the prism faces stand slightly in relief, and is waterproofed with shellac. "This prepared matrix (Figure 4)- two of these also are visible in an illustration on page 203-is now like a big round blank with which you would make a flat. It is so regarded and dealt with in polishing, figuring, and testing. Another flat, good to a tenth of a fringe, is used for testing it. Here there is, however, a complication-for plaster expands on setting, also changing with temperature changes, so that it is not a perfect matrix. This often causes much trouble. One must also learn to seat the prism so that errors will not be introduced to spoil the already corrected angles. "Polishing is done face up on a modified Draper machine. The polish must be free 3 of all defects-clear and grainless. "The two end faces are polished first, successively, then one roof face. "Before the second roof face is blocked, the roof angle must be corrected by hand grinding as closely as possible to 90 , so that when the prisms come out of the block, the angle error will be 2' or less, in order to save time later in hand correction. "Great care and correct abrasives must be used in grinding the final roof side. No tiny chips, nicks, or fractures are permitted along the roof line. Even No. 1000 Carbo will cause fractures here, and superfine emeries are used. "The second roof face now is polished (Figure 5). "Thus all four optical faces have now been polished, and next comes one of the most thrilling operations known to me in optics-correction of the roof angle by freehand polishing on a small flat lap of pitch. Correction of the roof angle is not in itself so difficult, but the lap must be held flat at the same time-flat within precision tolerances - without astigmatic fringes or turns. The crux of the operation is the preparation of the lap and the use of strokes which will produce the desired results-and, of course, a skilful man doing it. Such a man, one of the most skilled hand-correctors in the country, is Pavel Uvaroff, who has corrected as many as 55 roof prisms in a single day. Here is work where money and fancy equipment cannot substitute for the man. Figure 6 is Porter's sketch of one of numerous differing finger positions favored by prism correctors. "As the angle is being corrected, frequent testing is necessary. A target of crossed thin lines is placed about 30' distant and its image is viewed through the prism by means of a 20X telescope. The vertical lines do not double unless astigmatism is present in the prism. The horizontal lines double to a greater or less degree, depending on the angle error. The aim is to correct the prism till they coincide and are as sharp and clear as the vertical lines. With greater magnification we have proved that an error of 1" of arc can be discerned. "Readers who are already familiar with roof prism making will have found many omissions in the above hop- skip-and-jump outline. This is due to space limitations. I was invited to touch only the high spots, with the aim of giving the average amateur telescope-making reader a generalized picture. "One noticeable difference between mirror making and roof prism making- noticeable because, if the maker doesn't notice it, the purchaser's inspector certainly will-is the fact that the maker is no longer his own judge and jury. Any tendency to be optimistic about one's own work is cancelled out by the same cruel and heartless inspector. The prisms must equal specifications or else." BECAUSE it is anticipated that some readers will wish to go in for roof prism making, it probably will be a kindness-nothing less-to state here with entire candidness that it almost certainly is now too late to begin. Tooling up, alone, requires weeks or months, while learning the difficult art usually adds from three to six months more. It is not now believed: that this would result in much, if anything, more than a bad headache, irretrievable expense, and disappointment. In any case, the Scientific American Roof Prism Making Program is no longer open to new entrants-it has done its work. If this seems to be bad news, you may be tempted to ask why you were not invited to participate at the beginning, 18 months ago. Every effort was made to locate the ablest men, short of undesirable publicity, simply because every such man was actually wanted. Yet, despite a variety of under-cover fishing expeditions, many good men were no doubt missed. Requirements for admission were: several mirrors previously made, also flats. This led to some heartaches-the less the applicant had done, apparently the greater the heartache; in fact, several who applied without having made any mirrors at all nearly died of heartache. This seeming (and actual) exclusiveness arose from only one motive: the leaders found it impossible to service more than approximately 80 candidates. As a war was on, it was found necessary to be arbitrary-for which apology and the above explanation are now offered. Even as it proved, however, the leaders who tried hard to coordinate the work will probably never be the same again. Your scribe, after 18 months of preoccupation with the job, has selected a secret hole, acquired an eight-volume "History of the Dark Ages" (in which wars were fought without roof prisms), crawled in, and is about to pull the hole in after him -Goodbye! Emerging after a spell in the Dark Ages, he will, however, if the war situation or its end permits, endeavor to offer a second account in which details of the: program's accomplishments can be discreetly revealed. This will include a statement of the true identities of the heroes of the preceding story, each at present protected from sabotage under a name that, by censor's request, is fictitious. =================== Scientific American, February 1944: Jasset and Richards, also Gordon, were members of the recent "Roof Prism Gang." Because working only in spare time, of which there was not enough, they were unable to go into actual production but together produced 36 acceptable roof prisms and thus demonstrated that in more favorable circumstances they could have produced many. The initial belief on the part of your scribe, that the amateurs could make these prisms in spare hours, was mainly erroneous. The work proved to be too exacting for tired men who already had done one day's work. With one exception those who produced large numbers-thousands-were those who could arrange to quit their regular vocations and go into prism production whole time (actually, in fact, for most about double time, with sleep taken mainly after the emergency). The exception was the team of Ralph Franklin of Patchogue, N. Y. and Frank Cameron, Inwood, N. Y., who, together, produced 1700 roof prisms in spare hours- and felt about 100 years old during the long period of the doing. Today, all the roof prism work is completed. =============== Scientific American, September 1944: C. R. Hartshorn, 1244 W. 109th Place, Los Angeles, California... didn't want his name mentioned. He said he didn't want readers to think him one of those disputatious, contentious persons who miss no chance to jump on others. However, your scribe, who dealt with Hartshorn for a long time when he was in the famous Amateur Roof Prism Gang (where he wanted to make roof prisms gratis for Uncle Sam...) can testify to the contrary, =================== Roof Prism Makers, by Albert G. Ingalls. Telescoptics, Scientific American, November, 1944 THIS, NOW THAT it can be told, is the second half of the story of the recent Roof Prism Program of Scientific American's advanced amateur telescope makers. In the first account, published in the May 1943 number, it was told how Russell Porter, Alex MacTavish, and your scribe, had discovered in 1941 that the nation's need for roof prisms for military instruments was desperate. Roof prisms (Amici type) were suddenly needed by scores of thousands but there had been no peacetime demand and only two or three precision opticians had learned how to make them. They were about the toughest thing to make in optics; two of their faces had to be within two seconds (1/1800 degree) of a perfect right angle, or 60 times as close as the Porro prisms in binoculars and far closer than all but the most uncommon right-angled prisms. It was told how this magazine arranged with Frankford Arsenal to be the link between that institution and the amateurs, and how it organized 100 of the more able amateurs and throughout a year and a half coordinated their work while they progressed from single trial prisms to small and then large orders, working at home. Into this program Russell Porter threw the weight of his prestige, inspiration and all the help he was able to give. The third leader, MacTavish, had foreseen the nation's need for the difficult roof prism and, working alone and without written instructions (none then existed) had ferreted out the technique. He had patriotically turned over to the 100 amateurs, through mimeos issued by Scientific American all that he had learned, enabling them to start level with him. Who was MacTavish? "MacTavish" was the alias for Fred B. Ferson, Biloxi, Mississippi, which had to be assigned when the Office of Censorship, after going over the manuscript of the account of May 1943, said: Give all the roof prism producers fictitious names and, for fear of sabotage, omit their locations. "MacTavish's" (Ferson's) photograph has been in "Amateur Telescope Making - Advanced," page 337, for years. Thus the man who, out of patriotic motives, gave hundreds of tired hours to others and answered hundreds of letters while busy making his own roof prisms, was simply one of the nationwide fraternity of glass pushers- an average three-mirror amateur when he began making prisms. It is ironical that wartime necessity forbade revealing the identity of the amateur who gave the most to the Roof Prism Program. Here is a summary of the roof prism production of the amateurs-the prisms acctually accepted and purchased by Frankford Arsenal and three large wartime military instrument makers. Most of the names look more professional than amateur. This is because assuming formal company names proved easier than to convince a puzzled world that amateurs could make roof prisms. No conclusions should be drawn because of the large variation in production of the individual amateurs. Some unwittingly began almost too late; others lacked time or strength for the exacting work, which experience proved to be too strenuous for performance in spare hours. F. L. Frazine, jeweler, St. Petersburg, Fla., 18 prisms. F. A. Jasset, podiatrist, with F. Richards, Newton, Mass., 24 prisms. G. E. Gordon, photographer, Natick Mass., 25 prisms. F. R. Varela, engineer, Tenafly, N. J., 50 prisms. A. H. Johns, decorator, Larchmont N. Y., 165 prisms. G. Dallas Hanna, zoologist, paleontologist, California Academy of Sciences, San Francisco, with others of the Academy museum staff, made 165 prisms but shifted to emergency repair of Navy optical instruments. K. E. Dykoski, assistant in astronomy, University of Minnesota, 300 prisms. The Precision Military Optical Co., H. H. Selby, Encanto, Calif., 700 prisms. Selby later turned to essential war work in his vocation, chemistry. The Wellsville Optical Co., Wellsville, N. Y., David Broadhead (the "Jim Fogarty" of the May 1943 account) assisted by two part-time employees and by Mrs. Broadhead in the capacity of prism inspector, cleaner, and packer, food provider, and sympathetic listener to beefs and griefs individual and collective, 1850 prisms. Broadhead's vocation, doctor and bonesetter to the motion picture projectors of S. W. New York State, afforded him considerable spare time. The Emerson Optical Co., Inwood N. Y., Ralph Franklin, engineer, and Frank Cameron, assisted by Mrs. Cameron and one employee, were the only ones who made a large number of prisms (1700) in spare time, after doing full days' work in their vocations. For this they paid the penalty of creeping fatigue. The Optical Associates, of Detroit, L. H. Sampson ant several other amateurs, made 4500 prisms-which, however, was only a part of their general optical work. They plan to remain permanently in optics. The Modern Optical Co., Toledo 7300 prisms, consisted of Wm. Buchele, an amateur who had made a 20" reflector described in this department October 1939, assisted by his father, son, wife, and brothers, whom he trained to do some of the processes He remains permanently in optical work. Ferson-Linde Optical Co., Biloxi, Miss., 11,160 prisms, was Fred Ferson of Biloxi and Paul Linde ("Pavel Uvaroff'') of Crossville, Tenn., assisted by one full-time helper trained to do grinding. Ferson remains in optics as head of the Ferson Optical Co., Biloxi, while Linde, in Crossville, Tenn., is doing individual optical jobs on the side, as he had been doing before the war. Like the rest, these two pitched the time-clock into the well and worked from 8 A.M. till 9 P.M. till they were fagged-and then kept going. The total production mentioned above is 28,420 Amici roof prisms accepted and purchased at prices which ranged somewhat above and below ten dollars each. What percentage of the total roof prisms produced in the nation the 28,420 represents cannot yet be stated and is not large-probably under a tenth part. Where the amateurs made up for this, and more than made up, was in quality. Their percentage of prisms accepted by Government inspectors was higher than for all or nearly all the professional producers. This is not, however, a reflection on the professional. The amateur was running a small shop and doing his own work, or the finer parts of it, himself. He had high incentive-interest in optics as a science-and, above all, personal pride derived from his telescope making days. The professional seldom could employ men having the same incentive. In other words, had the amateurs greatly expanded they, too, would have had to employ wage earners for the finer parts of the work and then their level would have had to fall to the common level, which ran from 80 to 90 percent acceptance of prisms submitted; in a few instances higher. Not all the amateurs even equalled this level but most of them considerably exceeded it. Buchele, Broadhead, and others ranged around 98 percent. On their 11,160 prisms Ferson-Linde bettered 99-1/2 percent acceptance by purchasers. As Ferson pointed out in these columns in May, 1943, the telescope maker is judge and jury of his own work but when optics are sold to others a disinterested, tough, and hard-boiled, impersonal inspector steps in. Suppose, for illustration, the same rigorous standard were applied to the mirrors you make. For one single pit a mirror would be rejected, or for one tiny scratch visible only under a ten-power glass perhaps after you had been shown just where to look. How many mirrors would get by? That is what the prism makers encountered-greatly stepped up requirements (not a bad thing, however, since compromise would only beget more compromise, then laxity). The amateur had much more skill compared with the field-a major part of which consisted either of new firms without optical background or old non- optical firms thrown into optical work because it had to be done by somebody-than was sensed at the time. Had this been sufficiently realized nothing would have been allowed to obstruct a plan which Porter, Ferson, and your scribe at first projected: the gathering of the amateurs in one place and in one organization-Amateurs Incorporated. To this, two obstructions arose. One was the simple fact that, until the amateurs' own ability to do this optical work of the toughest kind equally well with professionals had been demonstrated-a thing which required so long a time that the demand for roof prisms was largely filled by the time that they knew they could successfully do it-they dared not quit their regular jobs and make the leap. A second was the fact that a certain professional optician was able to convince authorities that amateurs never could make roof prisms. The man who accomplished this questionable end did so at a critical time when the nation's need for roof prisms has been described as desperate. At best he used bad judgment. At worst his outlook was constricted. When World War III comes (compare with the history of Rome's three Punic Wars, in the third of which Rome virtually erased Carthage when Carthage was once more found to be rearming) let the amateur hit his optics early, hard, and unitedly, and with no inferiority complex. SINCE Ferson, starting with average advanced amateur background (three mirrors, a flat, a lens), devoted three years of intensive concentration to optics and learned a lot which daily work of the same kind afforded frequent opportunity to check, this department recently invited him to write out for the benefit of other amateurs and professionals the boiled- down juice of his experience; and since two chapters in "Amateur Telescope Making-Advanced" (those on the Gaviola and Zernike tests) had aroused little interest since their inclusion seven years ago, these have now been replaced by a single 29-page chapter by Ferson, entitled "Prisms, Flats, Mirrors," in a new printing of that book. In the same printing Dr. John Strong has completely rewritten his chapter on mirror aluminization and has included in it some new and valuable working pointers. The book is still Edition 1 but is now the fifth printing of that edition, and is revised to the extent of a total of 39 new pages. In his 29-page chapter Ferson gives the essential principles of prism manufacture for large- and small-scale production, also for hand production of small batches from a dozen down to pairs and singles-the singles being polished and corrected within annuli of plate glass for easier control by the tyro. He gives a list of prism-making equipment and some dodges for avoiding too heavy investment on small jobs, compared with the value of the prisms. He tells how to grind a prism to a close angle before polishing, a very pretty and labor-saving art. He explains how prism face angles are tested. He explains the exquisite art of correcting, freehand, the polished prism faces. This is the final process and the one calling for the greatest skill, but also is the most interesting one-rubbing off single seconds of are with a few simple strokes (simple if you know how!). For prisms, flats, and mirrors, Ferson lays much greater stress on grinding than our art has ever called for before. His intensive experience revealed grinding to be just as important as polishing and no longer a perfunctory merely mechanical, preliminary process. The practical dividend from supergrinding is the avoidance of such ills as turned edge. Ferson thus pursued one source of turned edge into its hole and dug up the hole- found the basic principle involved and devised a remedy, a part of which includes the use of a channeled glass tool for grinding, a talc-emery mixture, and drying up the wets. One hour per stage also is now held to be too little with Pyrex. The relation between prisms and flats may seem remote but actually is immediate: prism making is flat making. Prisms have flat faces but, even more to the point, they are blocked in batches in plaster matrices and thus are elements of a single big flat. Before the beginning of polishing, a flat can be ground within half a wave of the ultimate. For polishing, Ferson finds that scratches are not attributable to hard laps. He uses onion sack for sub-facetting and a rosin-pine tar mixture for the lap. The main secret of success is the use of the dummy polisher for keeping the lap itself flat: the same dummy is used in keeping a lap dead flat for hand correcting prisms. Ferson's work brought out the vital importance of correctly conditioning the rouge after applying it. Some rules of heat effects are stated. Finally, in his chapter, Ferson applies his prism and flat technique to mirror work, with the same channeled tool, same drying up of the wets, as precautions against turned edge inherited from grinding. But the dummy obviously can't be used on a non-flat surface. Mirrors are polished face up. Not all these methods are claimed by Ferson; some are old. Mainly they are modifications of our amateur technique which protracted, intensive work brought out as more important than we amateurs had previously realized. Some of them seem radical, yet 11,160 roof prisms with an acceptance percentage of better than 99-1/2 seems to commend them. Proof of the pudding Ferson now does professional work yet remains amateur at heart, still freely giving his experience to the fraternity, as in the new "A.T.M.A." Amo means to love. The amateur is one who works for the love of it. ====== (WE DISCOVER that the names of two producing members of the amateur Roof Prism Gang were omitted from the summary in last November's number. C. S. Walton, 5975 W. 44th Ave., Wheatridge, Colo., and Anton Bohm, 6815 W. 29th Ave., Denver, Colo., made 488 roof prisms. ) ====== (Dave Broadhead became ensnared in amateur optics in 1936. AS his first adventure he made three 4-inch optical flats and an objective lens, then a 6-inch Newtonian telescope, an 8-inch Dall-Kirkham and a 10-inch Maksutov. In 1941 he joined SCIENTIFIC AMERICAN'S wartime roof-prism group on learning that roof-prism-making called for "men who get results despite all obstacles." The first prism he made was so good that Frankford Arsenal asked for 50 more, 47 of which proved acceptable. In spare hours while doing other work he then made 2,850 roof prisms in the cellar shop of his home in Wellsville, N. Y. Government inspectors accepted more than 98 per cent of them. --1953) ============================================================== home page: http://www.europa.com/~telscope/binotele.htm