The Moving Picture World (April 1908)

MO THE MOVING PICTURE WORLD Lessons for Operators. By F. H. Richardson, Operator, Chicago. CHAPTER VIII.—CONDENSERS. Condensing lenses are made of standard diameter (4V2 inches) and of varying focus. The focus of the lens re- quired will depend on the throw and it is well to order the first pair from some reputable optical house, giving length of throw and size of picture. Find out exactly what it is they send and thereafter order the same; 6 1 /2- in. and 714-in. are most generally used and in some in- stances one 6V-* in. and one 7Y2 in- are used together. As to the grade of lens—it is a mooted question. The cheap 75-cent lens is most generally in favor and gives good re- sults but not quite so perfect as the higher grades. The more costly lens breaks just as readily, however, as do the cheap ones. As to condenser breakage, it is a thing that will occur tinder the most careful and intelligent management, but ignorance or carelessness will largely augment it. Briefly stated the main causes of breakage are as follows: (a) lenses fitting too snugly in the "round" (metal cas- ing into which the lenses fit in most lamphouses). Lens should never fit tight in the round. There should be at least 1-16 in. play and the ring should not be screwed down tightly. The lens should rattle when shaken; but this should not be overdone. There is room for the exer- cise of a little good judgment and common sense in this matter. If left too loose the lenses will not set square with each other, in which case the light will be materially deflected and much of it lost. Should the lens be too large it may be reduced by grinding the edges on a coarse grind- stone—never use an emery wheel as it will chip the edges and ruin the lens. (&) The light tpo close to lens, caused by lens of wrong focus—remedV: get right focus, (c) Circulation of air in lamphouse and condenser casing holes closed or vice versa. Remedy: regulate condenser vent holes according to amount of circulation in lamp- house, (d) Stoppage of screen over lamphouse with car- bon ash. This produces excessive heat in lamphouse with consequent abnormal heating of lens and liability to break- age. But no matter what you may do or how careful you may be condenser lenses will occasionally break. The writer has run for months without breaking a lens then, under seemingly the same conditions exactly, they would break, break, break, leaving him to simply scratch his head and wonder what caused it anyhow. One other prolific cause of breakage is allowing the carbons to flame. Carbons will "flame" when too far apart, especially if powerful current is being used, and this creates excessive heating in lamphouse. This is bad enough, but if the flame itself strikes the lens it will break sure. To recapitulate: Get your lenses of right focus for your work; have them fit loose in round; regulate your vent holes with judgment and don't allow your carbons to flame. By so doing you will reduce breakage to a mini- mum but—condensers will break and a stock should at all times be kept on hand. PROJECTION LENSES. Get them to fit your work. Get them of good quality. Get them of good size. Keep them clean. Supply the house that furnishes your projection lenses with exact dis- tance from lense to curtain and exact size (width) of picture you want and if they know their business they will do the rest. To find the size motion picture lens required divide distance (in feet) from lens to curtain by width (in feet) of picture desired and quotient will be number of lens. For instance: If a 15-ft. picture is desired at 60 ft we find 60 divided by 15 is 4—you want a No. 4 lens. This applies to motion picture lenses only. Always be sure your lenses are in exact focus. Nearly right will not answer. They should be exactly right. Select a scene with coarse grass or trees with leaves and bring out every spear or leaf clearly. Have some one manipu- late the adjustment screws for you and go yourself 'down into the darkned house close to the curtain and direct him. A lens may be nearly right but just a fraction of a turn of the screw may make it better, but you cannot perceive the difference from the op erating room. Lenses should be kept clean and this may best be done with wood or denatured alcohol, polish- ing afterward with a clean, soft chamois. Take the lenses apart occasionally and clean, but be very sure to get them together just as they were or you will have trouble. The smaller diameters of stereopticon lenses do not give nearly so clear-cut a picture as'those larger. A 2 1 / 4 lens is small enough (2% in. in the clear). But above all things re- member this: a cheap lens is an abomination and dear at any price—even as a gift ; The Picture Next. The Electric Light in. the Optical No. 7.—By C. M. H., in The Kinematograph and Lantern Weekly. Continued from page 286. Now that we have devoted some considerable atten- tion to the more technical part of the subject, it will be as well to turn our notice to the practical portion and deal with the matter from the point of view of the itine- rant lanternist arriving upon the scene of his evening's labors. First as regards the fuse. To refresh our memory, it may be again mentioned that the fuse is a kind of safety device inserted in an electrical circuit at different points along the wires, for the purpose of automatically breaking their continuity at any time, should the current become too large for them to carry with impunity. For the amount of current which you could draw from electric light mains would otherwise be only limited by the resist- ance offered by the work that you give it to do, and it might easily happen accidentally that that resistance was so small that an immense quantity of current would rush through the wires, with the probable result of burning them up, and doing considerable damage to the- installa- tion generally. A fuse is a piece of easily melted tin or lead wire, of small diameter in respect to the current that it is destined to carry. If from any cause this quantity of current is materially augmented, it will raise the tem- perature of that fuse wire above the melting point, and it will drop from between the terminals and immediately break the circuit and prevent the further flow of current So it will be seen that the first thing a lanternist has to see to on arriving at a lecture hall is that the wires bringing the electrical supply into the premises are suffi- ciently large to carry the amount of current that he requires for his lamp, and that the fuses that are inserted in the circuit are such that they will not melt under any legitimate strain that they will be subjected to during