International projectionist (Oct 1931-Sept 1933)

LIGHT AND LENSES Victor A. Welman SECRETARY, CHAPTER 18, AMERICAN PROJECTION SOCIETY II OUR first talk on lenses put forward but one principle of optics in addition to a few applications of that principle. In the preparation and presentation of this material it was my hope that I could stem the tide of wrong thinking in connection with lenses and their use and perhaps stimulate a new line of thought and re-direct this thought into the proper channels. It is bromidic to say that misinformation, even if sprinkled with grains of truth, is worse than no information at all, yet we see manifestations of the truth of this assertion in all sorts of printed works and in lectures. Once successful in directing our thoughts about lenses and light and how they and it are used in our daily projection work into the proper channels, it will be comparatively easy for the man with an inquiring mind and with the ambition to better himself to track down much valuable information on the subject. The result of our last talk, as indicated by the questions put forward, by some of the answers to these questions, and by various remarks made showed clearly a wide divergence of opinion. This is not strange, for after thinking of ink lines on paper as "rays" over a period of years, and after reading books and magazines which seemed to substantiate such opinion, it was too much to expect an instantaneous about-face. I am aware of the fact, of course, that many did not believe all I said in my last talk; and I am also aware that changes in habit of thinking about any subject must be brought about slowly. To those who still think that I advanced a "new" theory, and I know there are some who so think, I can only repeat that the basic idea of my thesis on light was first promulgated in the 17th century and that this idea still is presented in unchanged form in various textbooks. It was from these latest scientific and technical publications that the slides shown were made. The only originality was in presenting the ideas to a group of projectionists. In this talk I will add nothing in the way of principles. The fact that light ad vances in wave-fronts, and varies in speed in different media, accounts for all the phenomena in which we as projectionists are interested. From this point on we shall deal only with practical applications of light. The Light Train The light train from the arc to the screen may be divided into two classifications: (1) from the arc to the film, and (2) from the film to the screen. Endeavoring to attack this problem as nne continuous system has been the undoing of many writers on the subject. I have tried many times during the past 15 years to make this point clear to inquirers and have not yet succeeded. I shall try again tonight; and with the subject a bit more clear in my own mind, I may succeed; for there is much merit in the view that a failure to successfully explain a point to another means that that point is not wholly clear in one's own mind. I know that the idea that we must consider the light from the projection lens to the screen as diverging is quite general. Doesn't it so show up in this old stand-by (Fig. 1)? . . . diverging from the arc to the condenser; converging from the condenser to the projection lens; and diverging from the lens to the screen. Perhaps it has occured to some to wonder why a bi-convex lens used in front of an arc is called a condensing lens because it converges the rays of the arc onto the film, and then that a biconvex lens used as a stereoptican lens is said to diverge or spread out the rays all over the screen. We must remember, however, that we are dealing with wave-fronts and not with rays. That form of light with which we are dealing is shown in Figure 2. An illuminated point on the film sends out spherical wave-fronts, diverging, a small portion of which are intercepted by the lens and changed to converging wavefronts which come to a point on the screen. Figure 3 represents the pattern from one point on the film. Now, in your mind, fill in the pattern from a milliom Number 1 Number 2 NO. 5 points in a frame of film and then tell me where the rays cross! Since rays are wave-front directions and since wavefronts are crossing everywhere, the expression "crossing point of the rays'* loses its significance. Action of Lens Perhaps the custom of expressing the action in this fashion leads to the bad habit of thinking that the lens draws rays of light from the point in question. Of course, if we take the time to stop and think a bit we realize that the wavefront from the point really advances in all directions, as in Figure 4, and that the only purpose of drawing those boundary lines is to direct attention to the only part of the advancing wave-front that is acted upon by the lens. I have considered the system in its simplest form from an illuminated point on the film to a similarly illuminated point on the screen. Now let us see how that particular point can best be illuminated, keeping in mind the fact that we are interested in illuminating a million separate points on that film and not the frame as a whole. If we could develop some such method as is shown in Figure 5 by which each of the million points could be illuminated by a converging lens concentrating a minute brilliant point of light on it, we [18]