Moving Picture World (Jul-Sep 1911)

THE MOVING PICTURE WORLD 203 Projection Department Edited by F. H. RICHARDSON 3 QUESTIONS ON LAST WEEK'S LESSON. 1. — What happens when light rays pass through a prism? 2. Explain, in detail, what is illustrated by Fig. 30. 3. Of what use is shaded portion of the lens? 4. What is the process of annealing glass? 5. How is it done? 6. What happens if glass of a lens be not properly annealed? 7. What property of the lens would be injured? 8. Explain, in detail, how lenses are ground. LESSONS ON LIGHT (Continued). When the lens has been rough and fine ground and is ready for polishing, its surfaces have been rendered so smooth that it is semi-transparent. The process of polishing is similar to that of grinding, except that, instead of emery, sand or diamond dust, polishing powders of very fine grain, such as rouge and putty powder, are used, the mould being first covered with a thin layer of fine cloth or wax. The motion is the same, and is continued until all marks of grinding have been removed and a lustrous black polish obtained. The higher the degree of polish the less the loss by reflection. One would naturally suppose the lens to now be finished, and so it is, if it be a cheap lens, but the surface is still far from being sufficiently accurate for high class photographic or instrument work. The polished surface may be a slightly undulating curve, the irregularities of which may not exceed 1-100,000 of an inch, but in high class lenses even this infinitesimal variation must be removed. Such slight errors cannot be detected by mechanical tests, but by optical tests, dependent for their action on the interference of light, they are readily discovered. Slight irregularities are removed by a tedious process of local polishing with pitch polishers, on specially constructed machines. When one considers all this, an understanding of the vast difference in price between simple commercial lenses, and those for first class optical instruments, fine cameras, etc., is obvious. The mounting of the lens is another exceedingly important matter, and cheap projection lenses are very poorly mounted. The lens system composed of two or more factors must, for good work, be mounted so that the centers of the curves of all the lenses are exactly in a straight line with each other. This is accomplished by centering each lens in a lathe until the image reflected from its surface is stationary when the lens is revolved. Its edges are then trimmed to fit the mount exactly and as each factor of the combination is treated the same the whole system, when mounted in the tube, is exactly in line. Hard and brittle as glass is it is quite elastic and a lens may be strained so as to injure or change its refractive power by too tight a mount. The mount must be formed so that none of the image forming rays are stopped by any portion of the mount, either when entering or leaving the lens. The distance apart at which the lenses of a doublet are fixed is of much importance, and every individual lens of first grade must be treated separately in this respect. It is not sufficient that they be fixed a standard_ distance apart, as the least variation in thickness, of the individual combinations, calls for a special adjustment in this respect. This is usually only taken account of in the finer lenses, but it will readily be seen that the distance between the factors of a projection lens cannot be materially altered without seriously injuring the work of the lens. When one learns the minute accuracy necessary in the mak . ing and mounting of lenses for fine photographic and optical instrument work one is amazed that results so creditable •can be attained working with the cheap, carelessly made and carelessly mounted lenses used for projection. For the foregoing description of lens manufacture, as well as for much further information concerning lenses, I am indebted to a most excellent work "Photographic Lenses," by Conrad Beck and Herbert Andrews, published in London, England. This work is very good indeed, and I desire to give it full credit for knowledge obtained from its pages. But to return to the consideration of light action through lenses: In Fig. 30 we were shown the mechanical construction of a lens, or, rather, its mechanical principle; the same Bunch of Trouble. — McPherson, Kan., writes: "Here is a bunch of trouble for your consideration: (a) I have an Edison, Model B, machine and cannot keep the lower loop. Tension seems to be all O K. Sprockets are in line; tried new ones, but it did no good, (b) My picture works up and down. Tension springs are all right. Put in new aperture plate, but it made no improvement. I use no volt A. C, 60 cycle, through a Halberg economizer, 40 amperes at the arc. Throw is 65 feet, picture 17 feet and the "best in town." Use Bio and Electra, y% cored, top and bottom. The secret of good projection is The Moving Picture World." If your sprockets are in line and not worn the loosing of lower loop cannot, of course, be laid to them. Shove your lamphouse over and look at the lower loop carefully to see that it hangs straight; that is to say, that it comes down off the apron, around and up on the sprocket perfectly square. If it doesn't, then the trouble probably lies to some extent there. I do not, at the moment, remember whether or not the Model B has a stripper plate. However, by close examination you will find a way to square the loop if it is out of true. Look at your lower sprocket idler and see that it is not riding on the film with pressure. If the idler bears heavily on the film it will likely cause it to climb, thus loosing the lower loop. When your lower reel is almost full the take-up tension ought to be barely enough to revolve the reel. Anything more is excessive and likely to pull out the lower loop when bad patches go through. You must remember that, when the run first begins and the diameter of the lower film-roll is small, a very little excess of tension means heavy added pull on the film, since the leverage then is far greater than after the film-roll becomes larger, (b) Watch the picture carefully and if the movement up and down occurs four times to each turn of the crank then either your intermittent sprocket is not true or its shaft is sprung. Remove the sprocket and shaft from the machine. Take sprocket off the shaft, put shaft in a lathe and test it. If it is out of true, order a new shaft. If it is true, order a new sprocket. Remember that you are dealing with minute measurements, however. Everything in your 17-foot picture is magnified 204 times (linear magnification, I mean), so that if your sprocket or shaft should be out of true even 5-1000 of an inch, it would mean a movement of a trifle more than one inch on the screen. If the trouble is not found there please write again, describing the movement very carefully. Glad to hear report of good projection in McPherson. Keep up the good work! Bad Insulation. — Cristobal, Canal Zone, Panama, writes: "Find enclosed $1.50 for handbook. Kindly see that it gets on first steamer. Recently we were troubled with flickering of the light and followed the various suggestions in Projection Department without effect. Finally we discovered the mica insulation had become worn thin on one side, possibly causing an intermittent short circuit by jumping of the current. Since renewing the insulation we have had no further trouble. This may, or may not have been mentioned in the Department, but we send it in for the benefit of others, in case it has not." We are obliged to the Neighbor for contributing something not before touched upon. Such a condition might possibly arise, provided (a) the lamp be grounded; (b) the insulation of the other arm be bad also. We have been having some tropical weather up here, Cristobal.