Moving Picture World (Nov-Dec 1923)

P P^O J E C T I EDITED BY F. H. RICHARDSON Louisville , Ky. There will be a reasonablj^ warm time in Louisville, Ky., next New Year’s^ Eve, for the Louisville Motion Picture Projectionists, Local Union No. 163, I. A. T. S. E. & M. P. M. O., will give a Grand Ball and Carnival, which it is expected will be the most sizeable event pulled off in that neck o’ the woods for quite some while. And that’s saying considerable, because you know when those Kentucky men and their fair ladies start out to do something, that thing is always well done. Harry F. Petty is chairman of the entertainment committee. Petty is projectionist at the Rex Theatre, Louisville. I wish you every joy on New Year’s Eve, gentlemen. Only wish I might be with you and help roll the joy ball until the “wee sma hours. Can’t be did, though — too far away : Blade too Narrow Joe De Reny, projectionist, Grand Theatre, East Stroudsburg, Pa., wrote us under date of September 19, saying: Have been projecting with Simplex projectors for almost a year and have had no trouble at all until now. The rotating shutter is set almost as perfectly as possible, but it certainly does pull up and down on the screen. Lines show up and down on reading. and even on the picture, epecially when I speed up a bit. What is your advice in a case like this? Two-blade shutter; alternating current. Now see here, Old Timer, without the slightest intention to “roast” you, I must say that if you would invest a few dollars in a Bluebook and STUDY IT, you would have no need to ask questions such as this. I am, of course, glad to help you (though somehow your letter seems to have been mislaid and overlooked for quite a spell, for which accept apologies), but I would far rather see you able to answer such a question yourself. You will find all such questions answered and fully explained under “Revolving Shutter,” pages 611 to 624 of the Bluebook. Your specific question is answered on page 624 and the remedy is given. If you have travel ghost (white streaks up or down, or both, from white objects on the screen) both ways, then your shutter either has too much lost motion or its master blade is too narrow; probably the former in your case. Hold the flywheel stationary and see how much you can move the rim of the shutter. If the movement is excessive, there either is some gear loose on its shaft or some gear or gears badly worn. If the latter, then I think you had better have a loan head sent you and ship your mechanism to the factory (317 East 34th Street, New York City) for overhauling and repairs. If there seems to be no excess of lost motion, then hold a sheet of dark colored paper in the light beam in front of the projection lens, and at varying distances therefrom, to betermine the point where the light beam has least diameter. Perhaps you may be able to move your shutter in or out on its shaft to a position where the light beam has less diameter. The aerial image is the best place, if you can get the shutter to it. If nothing can be done this way, then you must have a wider master blade, and you can get one by either using very small rivets, attaching a bit of metal to each side of the master blade — each side in order to preserve the balance — allowing it to extend over the blade edge sufficiently to add enough width to stop the travel ghost. For myself, I would cut a master blade of stiff cardboard, such as business cards are printed on, making it the required width, riveting it to the metal master blade. The thing is to make the blade a bit wider and to do it without seriously disturbing either the mechanical or the optical balance of the shutter. I advise you, strongly, to get a Bluebook and study up on such things. In the end it will prove the best value you ever got for six dollars, and the fact that you ask this question is evidence that you need it. Condenser Argument John A. Maurer, projectionist, Massillon, Ohio, says : A matter which has an indirect, but a nevertheless important, bearing upon your recent discussion with John Griffith with regard to the condenser line-up of Bruce S. Watson, Muscatine, Iowa, is the thing I shall discuss. With the piano convex or M. Bi-C. condenser systems now in general use, in which both lenses are of the same diameter, the screen illumination does not fall off nearly so rapidly when crater distance is increased as the ordinary method of calculation would indicate. This does not mean that the method of calculation is wrong, but that other considerations enter into the matter in such a way as to modify its application. The accompanying diagrams are accurate copies of drawings made by Young’s construction — an accurate method of laying out light ray action which takes care of all factors entering into the matter. These diagrams are based upon what I know to be the actual curves of the imported lenses now on the market, assumed to be made from glass having a refractive index of 1.525 — which I have found to be the average of a number of specimens tested, and, to be strictly accurate, on yellow light. Figure I illustrates a combination of two 6.5 piano convex lenses spaced 1-16 inch apart, with the crater three inches from the plane face of the collector lens. Figure II shows the combination of a 6.5 and an 8.5 inch piano convex, with the crater four inches from the face of the collector lens. Figure III shows the line-up of the Muscatine projectionist. It is of no especial importance in this discussion, and is included merely for your comparison. By the ordinary method of calculation the light-gathering powers of systems I and II would be in the ratio of the square of 4 to the square of 3, which would mean that system II would pass 44 per cent, less light than would system I. However, you will note that in neither of these systems is the whole of the diameter of the collector lens effective. You will also note that in figure II a larger diameter is working than is effective in figure I. This is partly due to the rays being more nearly parallel between the lenses and partly to the fact that the thinner converging lens reduces the distance between the edges of the two lenses. The angle of rays effective, insofar as concerns the converging lens, is 59 degrees in system I and 52 in system II. Now, in order to accurately calculate the relative amounts of light in these different angles, it is necessary to compare the areas they intercept on the surfaces of spheres (imaginary) of equal diameters, having their centers at the source of light. The simplest way of getting this comparison is to compare tlie values of one minus the cosine of half the angle, or as a mathematician would put it, “Vers. A.” Figured by this method system II PASSES 22 per cent, less light than does system No. I. And now here is another factor to consider. No. II has much less spherial aberration than has No. I. The importance of this factor in condenser systems has. I think, been greatly underestimated. Its effect here is that a part of the extra light which does get through No. I is wasted at the spot, or if it does appear on the screen it is as discolored light. This effect is so pronounced that it very nearly evens matters up. as between the two combinations. The light from No. I might be a bit the brighter of the two, but that from No. II certainly would be the whiter and more pleasing, and this, be it understood, is entirely aside from any question of getting the entire light beam through the projection lens. Figures l-a, 11-a and IH-a show, by pairs, the crossing points of marginal and paraxial rays from combinations I, II and III respectively. They are drawn to the same scale as the other diagrams, i.e., full size. The scale of inches under each d.agram has its zero at the center of the combination from which the rays are coming. If you