Moving Picture World (Nov-Dec 1923)

December 1, 1923 MOVING PICTURE WORLD 50 7 extend the lines of these figures the proper distance to the left you will find they will join up with the rays of figures I, II and III. I think these drawings really speak for themselves better than I can speak for them, so I will close with the remark that I have been studying this angle of the condenser problem for several months past, both theoretically and by practical experiment. By the way, I might remark that diagram III-a shows that either Muscatine has not given his exact crater distance, or his condensers are inaccurate as to focal length. He says he uses his aperture 18 inches from the aperture, whereas the line-up he gives forms the best spot at 15 inches from the condenser. Note: I do not claim that any of these diagrams represent actual exact working conditions, but this does not in the least affect their value as studies of the effect of spherical aberration. First off, while I think of it, when writers speak of distance from condenser they should always designate whether its optical center, a point midway between the two lenses, or the face of the collector lens is meant. As to what you have advanced: Well, for the most part, I shall have to leave the discussion of it to those better versed in scientific optics than I am, maintaining, however, that while it is possible that other elements, such as those you have named, may operate to modify the operation of the law of the open light source, as illustrated on page 162 of Bluebook, to some extent, as I now see the matter I would place heavy emphasis on the “to some extent.” Figure I-a, Il-a and III-a are interesting and instructive— very much so, in showing the varying effect of spherical aberration in different piano convex and in the M Bi-C condenser. If your drawing III-a is correct, as I presume it is, the reduction of spherical aberration by the addition of the two additional surfaces is much more pronounced than I had supposed it to be. Let us see what some of you optical sharks can do with what brother Maurer has presented. JUST OUT A Brand New Lens Chart By JOHN GRIFFITHS Here is an accurate chart which belongs in every projection room where carbon arcs are used. It will enable you to get maximum screen results with the equipment you are using. The news Lens Chart (size 15" x 20") is printed on heavy Ledger Stock paper, suitable for framing. It will be sent to you in a strong mailing tube, insuring proper protection. Get this chart now and be all ready to reproduce with maximum screen results the splendid pictures which are coming this fall. Price $1.00 Postpaid Chalmers Publishing Co. 518 Fifth Avenue New York City Why the Sputter An Iowa man, who desires to be unknown, is up to his eyes in trouble. He says : I have been projecting motion pictures in this small town for a year and have had no trouble at all. Have the Bluebook and had the Third Edition as well, and have studied hard. To that and the kindly assistance of the boss I attribute my ability to get by. But now I’m up to my eyes in trouble. I am using the same carbons I always did — Columbia— but of late the arc continually sputters and I just can’t hold it steady. It has nearly driven me mad with worry. I built a film storage cabinet a few weeks ago and it works just fine. Have a separate compartment for each reel, with a space below which I keep filled with water. Above the reels is a cupboard in which I keep my supplies. It is very neat and convenient. Really, friend, I had to laugh. You built a film storage tank with water below and your supply cupboard above. Keep your stock of carbons in it too, don’t you? The trouble (sputtering) began just after you got that cabinet finished, didn’t it? Sure! The water reservoir you have kept religiously supplied with very wet water, which is good for the films. BUT when those carbons sputter they are merely swearing at you for subjecting them to a vapor bath and then expecting them to burn well. Your cabinet is doubtless very nice, and all right for the film, but you yank those carbons out pronto, and put them in a thoroughly dry place ; also place half a dozen of them in the bottom of the lamphouse to dry out, and every time you use one replace it from your stock. Do these simple things and see how quickly the carbons will stop sputtering, and start purring in satisfied content. See page 388 of Bluebook. He Has Trouble Jacob R. Steinfield, New Orleans, La., has trouble on his mind, which he describes as follows : I have two Power 6-B projectors, am using fifty amperes from a motor generator set and have been getting what I thought was very good results until right lately. Two weeks ago a projectionist who had a card from the Chicago local visited me and talked very big about “what they did in Chicago.” He was not at all slow in telling me what I ought to do, one thing being to take my projection lenses apart and clean their inner surfaces. I had never done such a stunt. The lenses looked clean and I hesitated, whereupon he took one of them apart and “showed me how.” He went out and got a little alcohol, mixed it with water and washed all the lenses, and cleaned everything thoroughly. I’ll say he did a thorough job, BUT when I started the show that night, and Chicago had departed, there was a ring of light clear around the screen when the projector of which he cleaned the lens was working. The other was all right — its lens not having been touched. I have taken the lens apart and, so far as I can see it is all right. Can you tell me what 'it is and how to fix it? The trouble is that your Chicago friend did too thorough a job, or scrubbed one wrong place rather. Evidently he wiped the interior surface of the barrel, which he should not have touched, and in so doing removed the black coating, or some ot it. The ring you see is light reflected from the sides of the lens barrel. Take your lens apart and paint the interior of the barrel with a coating of coach painter’s lampblack thinned down with turpentine, allowing the paint to dry very thoroughly before re-assembling the lens. In other words, be very certain that all the turpentine has evaporated before re-assembling. You may hasten this by heating the lens barrel SLIGHTLY — just so you can still hold it in your hand without dis :omfort. Or you may lay the barrel in the blast of a fan for half an hour. In re-assembling, be very certain that you get each lens in the same place you removed it from, and that the strongest convex surface is next the screen. Set up the holding ring snugly, but DON’T clamp them down tight on the lenses. You should take your lenses apart and clean their interior surfaces after about five hundred hours’ use, if you run say ten hours a day; or after two hunded and fifty hours’ use, if you run evenings only. They may look perfectly clean as you look through them, but that is no proof they are not sufficiently fogged to greatly increase the reflecting power of the surfaces, hence the light loss. See page 141 of the Bluebook. Do not imagine that that chap represented Chicago. Chicago has some AWFUL dubs, true; also she has some as good men as you will find anywhere — distinctly high class. Most large cities are the same. His Difficulty (In last week’s issue an Ohio projectionist complained of his eight-year-old lenses — chief difficulty being curvature of field. I strongly advised him to get new lenses. Due to an accident in the print shop, the additional information which follows was left out of the paper. — Ed.) Of course, the installation of new lenses would effect no startling change. The patrons might not even notice it, and the manager might have to look twice to see the difference, BUT THE DIFFERENCE IS THERE, and the thing friend manager has to sell HAS been improved, hence, while the difference in seat sales may not be at all noticeable, it nevertheless is there, since the better a thing is the more inclined the public is to buy it. As to Lenses As to the possibility of absolute flatness of field, there is only one kind of lens will give that, and that is an anastigmat lens, BUT with the present day high-grade projection lenses available, there should be no perceptible curvature of field with a sixteenfoot picture at 120-foot projection distance; also you should be able to get an acceptable sharpness at top and bottom without your screen titled, with the projection angle you have, or at least a very slight tilt should do the trick. I would suggest that you buy direct from a lens manufacturer and get a guarantee that there will be no perceptible curvature of field. As to a standard — I know of none which could be applied, except this : Cut a strip of mica the width of a film. See to it that it is perfectly flat. On one surface draw perfectly straight scratch lines one-eighth of an inch apart, both ways. Place this in the projector in place of a film, BEING CERTAIN IT LIES PERFECTLY FLAT OVER THE APERTURE; project the lines to the screen and test their straightness with a line stretched over them. If the lines are not straight the lens is imperfect. You may be certain the mica is flat over the aperture if the lines are in sharp focus all over the screen. If any one can suggest a better method of lens testing, let him do so. Get Your Christmas Copy of Richardson’s Latest Handbook on Projection Now $6.00 Postage Prepaid