International projectionist (Jan-Dec 1935)

May 1935 INTERNATIONAL PROJECTIONIST 47 cally dissent. He says that certain advice (on projection) is of little value because so few men will heed it. This thought, carried to its logical conclusion, would mean inefficient projectionists and generally poor projection — a clear case of "Oh, what's the use?" Mr. Richardson must admit that at least one man has benefitted by my article, and that man is his correspondent, Mr. Schrock. The latter says that he has fitted brushes by pulling the paper both ways for a long time — maybe for years — and he apparently has not given the matter much thought. But, and this is the important point, he read my article and started to think about the subject. "Maybe Schroeder is right and I am wrong," he might have thought. In any event, Mr. Schrock went to the trouble to write a letter to Mr. Richardson and ask his views on the matter — an act which definitely bespeaks keen interest. The chances are that Mr. Schrock now will go to a bit more bother in fitting his brushes. He may even go to the trouble of following Mr. Richardson's advice and pull the brush out two or three times in order to sand the high spots. At least he knows more about the subject than he did before he read my notes, even though they appear to be "too finely spun" to suit Mr. Richardson. — A. C. Schroeder. LIGHT AND LENSES (Continued from page 15) boundary of a wave-front coming from one corner of the screen; it comes to a point at the corner of a frame of film. Now, to get the same amount of illumination at this point as at the center, this wave-front should continue to advance along the same line. See where it lands. In order to furnish that illumination a 12 or 14-inch condenser would be required. A condensing lens of that diameter of the proper focal length to keep the arc within 4 inches of the lens (which must be done to get proper efficiency) would be so thick that spherical aberration would make impossible its use. Reflector Arcs Now, perhaps, you get a clearer idea as to why the reflector arc is so much more efficient than the straight arc. In handling wave-fronts the mirror acts just like a lens, except that the light passes through but a very thin thickness of glass. There is no difficulty experienced in making a mirror of 6, 8, 12, or 14 inches in diameter. The trend toward reflector arcs is thus inevitable. First we had the 20-ampere lamps, then the 30-, 60-, and 70-amperes lamps. The next development will be 90 and 100ampere reflector arcs. J~ *'"' L_ Figure 21 • that every Projectionist may know • S. O. S. 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Corp. 1600 Broadway, New York, N. Y. Z?ZZZZZZZZZZZZ!cZZZZZ The question of whether to use a 6lA or 7^ focus, or any other focal length piano convex 4M>-inch diameter condenser, has become only of academic interest, due to the introduction of the reflector arc, and is no longer of interest to us. Previously it was almost wholly a matter of "cut and try." It could have been reduced to a certainty, but it never was. All that I have ever read approached the topic from the wrong angle, the theories upon which these expositions were built were fundamentally wrong, and the writers thereof endeavored to find an explanation for what was found in practice to be correct, instead of trying to determine theoretically what would be best and then attempting to reduce that determination to practice. Bausch and Lomb undoubtedly dug into the subject from a scientific angle and, instead of trying to determine which of the popular combinations was best, found that none of them was even fair. They then began developing their Cinephor series of condensers. Let us get back to the projection lens for a moment. The Petzval Type Lens In 1840 Petzval brought out a portrait lens which worked at the remarkable diaphragm (for that period) of f/3.5, when at that time f/60 was considered good and when, in fact, portraits were very difficult to take because the subject could not sit still long enough to allow for sufficient exposure. I can't go too deeply into apertures just now, but suffice it to say that for a lens of 6-inch E.F. the free opening of the Petzval would be 1.7 inches, and of the f/60 it would be .1. so that the Petzval would have 17 times the diameter and would pass about 290 times as much light. Owing to the large relative aperture, the Petzval was immediately adapted for lantern work. Figure 21 is a sketch of a Petzval lens taken from a book published about 40 years ago. This lens consisted of a cemented front combination of flint and crown glass and a back combination of flint and crown separated by an air space, the combinations being separated by a distance depending upon the focal length. Let us look back to the state of science and industry in 1840. The arc lamp had not been invented; automobiles, telephones, and phonographs were missing. Railroads were just being developed, and the electrical industry as a whole was not even dreamed of. In thinking of the great development in other lines one might expect the projection lens to develop apace. Let us see. The lower half of Figure 21 was taken from a circular received from an optical company1 describing their newest development in projection lenses2. Ninety years have not changed the fundamental