Moving Picture World (Sep 1916)

1830 THE MOVING PICTURE WORLD September 16, 1916 one another, and so operated that they open and close the circuit with a very quick break. All of which is passed along to Brother Grey, and others to whom It may be of value. I would suggest, however, that the spark-plug diagram would be quite acceptable if Neighbor Le Brun will kindly supply it. Optical Center. George Raymond, St. Louis, Mo., propounds the following : i Is the optical center of the lens always between the two surfaces of the lens? No, it is not. To find the optical center of a meniscus lens you would first measure the curves of the lens, and then make a circle of such diameter that its periphery would fit the surface of the lens. In the meniscus lens, which we take for example, we would draw two circles, as per accompanying sketch, one to fit the concave and one the convex surface. From the center of these two circles marked AA, draw lines BB, which must be exactly parallel to each other, let one line extend to the concave, and the other to the convex surface, and then draw line C as shown. The optical center of the lens will be at D. Light Trouble. E. D. Kiddoo of the Nebraska Conservation and Welfare Commission, Lincoln, Neb., writes as follows : You have been recommended to me as an authority on motion picture projection. I therefore wish to ask you a few questions. Our machine is a Power's Six mechanism with a Six A lamphouse, mounted on a wood base so that it is portable. We meet most all conditions as regards current, length of throw, etc. Lenses are 2, 2y2 and 3 inch, and we are at present using 7% and 6% inch condensers. We have been having a great deal of trouble with blue ghosts in the field. The maximum distance from the condenser nearest the aperture and the aperture plate is 18 inches. We use % inch Columbia cored carbons, adjusted about as per sketch, the current being alternating. The condensers are about % inch apart, and the distance from the crater to the condenser is from 2V2 to 3 inches. When the arc is moved back in the lamphouse so as to eliminate the blue ghost the field is not well covered in the corners. Are the carbons set right for A. C. ? Are the conoensers of the right size and distance apart? For example : 'Using a 3 inch lens at 100 feet, what size picture would be projected, and in what position should the lamphouse and arc be to cover the field well, and eliminate the blue spots? Have you tables showing these things? In setting carbons what have you found to be the best way to arrange them for A. C? For D. C? Is there any difference? And now as tothe shutter. Should one use a three or two wing shutter with A. C? With D. C. ? In timing the shuter should the wide blade just cover the lens when the intermittent begins to move, or should it be two-thirds or one-half of the way across? Do you know of any belt, leather or otherwise, which will successfully operate the take-up when using 2000 foot reeis .' We are having trouble in that a tight belt pulls out the lower loop when the 2000 foot reel is about two-thirds through, and if too loose it does not take it up. Do you know of any company which puts out a good cloth screen that can be folded and carried around, and one which has no seams in it? This screen should be 12x15 feet at least and should be heavy enough to work well out doors without any background. No doubt these questions seem simple to you, but the solution of some of the problems would help us in our work. No, the questions you present, Brother Kiddoo, are not at all simple. In fact, it lias in the past cost a great amount of labor and money to ascertain the correct answers to some of thoni. To answer these questions fully and intelligently would require at least six pages of this department. I therefore trust you will not feel that I am trying to force you to buy a Handbook when I do the only thing x really can do, viz : refer you to the answers to your questions as set forth in the new 720 page Handbook, price $1.00, from the Moving Picture World. First : 1 would refer you to "Carbon Setting," pages 200 to 302, inClUSive. I would refer you especially to Figure 123. page 205; Figure 124, page L"1", and Figure 126, page 300; also I would recommend that you carefully study thi text matter. You are using tue antiquated, out-of-date Jack knifi et, with which it is utterly impossible to semi, steady light, With the set as shown in Figure 126 you win not. gel quite so bright ;i light, bul you will gel a steadier light after you have learned to handle It, and the brilliance can ho secured by increasing tho amperage. M i po slble thai a pari et' the discoloration you speak of is due to the set you are using. \ in Hie lens proposition, I would refer you to pages 113 to lit;. inclusive, ami particularly to Table L, page Ml. which gives you all the data you ask for. It will be li tor me to tell you what con densers you want, and what distance you need between lamphouse and film without knowing the back tocUS of the objective and its exact aeter. As to the distance between the condensers, the altering of the distance between the lenses operates to change the E. F. of the condenser. Except in special instances, where it is necessary to change the E. F, of the condenser without having the proper lenses to do it, I would recommend that the two eon, busing lenses be placed so there is not more than 1-16 of an inch between the apex of their curved surfaces. As to the two or three-wing shutter, you will find that fully dealt with on pages 469 to 476 inclusive. If you will note the number of pages of the book included in each one of these topics you will see why I could not possibly answer all this intelligently in the department. If there is not more than the ordinary lost motion in the gears of the machine the shutter should be set so that it covers about two-thirds of the lens when the intermittent is just on the point of moving. I would also recommend that you examine Figure 63a and Figure 1-45 of the Handbook, together with the text matter on page 146, and thus learn how to find the proper distance to set the shutter from the lens. There should be no trouble with your take-up belt pulling out the loop with a 2000 foot reel, two-thirds rewound, and if there is trouble such as you set forth it ought to occur right at the beginning of the run. It seems to me as though the spindle of the take-up which carries the lower reel is not properly lubricated. Better take it out,, examine it, and see if there is anything wrong with the bearing. As regards the cloth screen, I am not sure, but I believe the J. H. Genter Company, Newburgh, N. Y., can supply you with a cloth screen of the width you seek; also possibly the Mirror Screen Company, Shelbyville, Ind., can. It is also possible you may get it from the Minusa Cine Products Company, 2665 Morgan street, St. Louis, Mo., but outside of those three places I don't personally know of anywhere else you can secure cloth of a greater width tnan 10S inches, which latter is carried in stock by most large department stores. You can, of course, obtain a Mirroroide, a Minusa, or any of the patent metallic surface screens of that width which may be rolled up and carried along, though they cannot be folded. Now, Friend Kiddoo, please remember that I would be glad to answer all this through the department if it were practical to do so. The thing you need, however, is the Handbook. Send §4.00 to the Moving Picture World, 17 Madison avenue, New York City, and it will be sent you at once. An Interesting Letter. W. H. Booker, Chief of Bureau, North Carolina State Board of Health, Raleigh, N. C, writes : First of all, I wish to thank you for calling our attention to your most excellent Handbook for Managers and Operators. I think I have read everything in that book that applies to our outfit, and part of it I have read several times. I have also had our lecturer and operator read it, and I wish to assure you that it is of tremendous assistance to us in our work. 1 have also written to the Wallace Golden Rule Shows, at Red Oak, Iowa, and have received two of the most courteous, helpful letters imaginable. I think it is a bit unfortunate that some of our friends in the moving picture business seem to think they have a corner on the information in regard to moing pictures, but not so with such men as are back of the Wallace Golden Rule Shows. They do not hesitate to show their hand and offer assistance in every way, shape and form. As I wrote you in a previous letter, our outfit is equipped with a generator set made by the Universal Motor Company of Oshkosh, Wis. The generator has a capacity of 50 amperes at 60 volts D. C, and by using % inch cored and % solid carbons, and a Power's Road equipment we have been successful, in our own opinion, in securing a fairly satisfactory picture on the screen. Ordinarily, in little country school houses, we use a ten foot standard muslin screen, although we have a sixteen foot screen for larger halls. I should like to ask your opinion with regard to the matter of resistance in series with the arc. At present we are using two grids from a Power's grid rheostat, and these two grids we have located in a Studebaker car and pass the current through them in parallel or in multiple, after which the current goes through either a 35 foot piece of No. S stage cable, or a 65 foot piece of No. 4 stage cable, or a combination of the two. depending upon the distance we have to locate the car from the projection machine. We very rarely have to use both pieces of cable except where the hall is on the second floor. The manufacturers of our engine generator set recommend using tho outfit without rheostat in series. Neither do we use the field rheostat with the generator. The current for the field is taken directly off the armature, although it would be readily possible to install a rheostat for the field if it were necessary. Would you recommend dispensing with our two grids, or making some other combination, or what would be your advice in regard to a ballast or steadying resistance in scries with the arc. We have been having some little trouble breaking condensers, but I have recently ordered one of tne Preddy mounts. Sometimes in very small schoolhouses where wo have to use a 3 inch projection lens, wo find that we are scarcely able to get over a 4x(i picture, and in a great majority of cases we are scarcely able to sot over a 4x0 picture, and in a great majority of cases wo llnd that a 10-foot picture would be amply largo. It has occurred to mo that we might, in all probability, be able to use a %-inch cored carbon with very good results in such cases. Don't you think such a carbon would be ample largo'.' If. in your Judgment, a smaller carbon would be satisfactory, please advise mo whore they can he secured. I should like to ask your opinion in regard to an asbestos booth. I nolo the Howo shows use an asbestos cloth arrangement over a metal frame. We arc endeavoring to travel just as light as possible and. in fact, wo already have our Studebaker car very much overloaded. I should like very much to cut the weight of our equipment down materially, but. of course, we do not wish to seriously endanger anyone, and as a rule, it is neces