Motography (Apr-Dec 1911)

October, 1911. MOTOGRAPHY 175 know how, or does not take the trouble necessary to. correctly place the lens. But the operator must not be blamed in many cases, because, in the first place this adjustment is always somewhat difficult to make; and in the second, a large number of machines are sent out so badly arranged, that without the use of a file and a hammer, no approach to exact centering can be attained. This is not very important where few lantern slides are made use of, but it does not add to the attractiveness of a performance to present any part of it in second-rate style. The optical center of the projection apparatus is an imaginary straight line, one end of which is at the center of the picture on the screen and the other end at the center of the crater in the upper carbon. Starting at the carbon end, it passes through the center of the condenser, the center of the gate aperture, the center of the two back lenses of the projecting-lens, and the center of the front lens. The screen should be at right angles to this line, the projecting lens and the gate also. The exact squareness of the condenser to this line is not of rigid importance, a few degrees of misplacement will not affect either the definition of the picture or the even illumination of the screen. The arc being adjustable in all directions, can always be placed in line, and has to be so in order to get the best illumination. The above statements must be taken to some extent as being the theoretical conditions to be aimed at; unfortunately, in many machines, now in use, it is impossible at all times to arrange that the above stated conditions can be complied with. Perhaps it is correct to say that the masking arrangement on more than 50 per cent of the projectors prevents the possibility of keeping the picture on the film always centered, optically speaking. In those machines where the mask is moved up or down to suit the picture, it is evident that only in one position can the optical centering be quite correct. When the mask is fixed and the film can be moved to suit its pictures to the mask, the adjustments remain correct. In using a machine with movable mask, its best position should be found by testing (as will be presently explained), and a mask made so that the latter can be centered quickly. The mask should always be brought to this position when threading up, and the film adjusted so that the picture is correct in the nearest possible position to this point. I am always asking for, and harping upon the advantages of standardization of the perforation of the films. Here is another point where, if we could have our films always perforated so that either a hole or a space would come exactly at the joint of two pictures, no masking during a performance would ever be necessary, the projector could be better and more simply constructed, in that the shutter could be somewhat smaller, and in many machines the shutter mechanism could be less complex, the automatic cut-off more easily made, and last — but not least — the optical center would be always correct without necessitating the extra mechanism now often employed to cause the film picture to move to suit the fixed mask, and the operator would have one less adjustment to bother with. To test the centering of a projector proceed as follows : — Take the condenser lenses out of their mount and unscrew the projecting lens cells and take out the lenses. Get some thin cardboard and cut out a piece that will just fit into the condenser tube (it is easy to mark the size with a pencil, using the mount as pat tern) ; also cut pieces to fit in the cells of the projection and lantern lens. Then with a pair of compasses mark the centers in all the cards while they are in place in the mount and cells, using the compass with one leg against the inside of the cell or mount. Make four marks, each a little short of the card, and with a thick round pointed tool, pierce a hole in the exact center of each card and enlarge it to about % inch diameter, keeping it quite central by means of the four marks which should be far enough from the center to be quite clear of the hole. If only a center mark be made, one is never certain that the hole is not deflected ; but it is easy to keep quite central by watching the four marks and pressing the tool accordingly. The stiletto from a lady's work-box is a fine tool for the purpose. Next cut a piece of thick card and fit it exactly into the mask aperture, and make a corresponding hole in it. On placing the condenser mount in position and also the projecting lens cells, the holes in the cards should be all in the same straight line. The mask may want moving up or down to make its card hole agree, and then a decided mark should be made for setting the mask on center. If a new machine will not stand this test it should be returned to the maker for correction, and any machine showing defects under this test should at once be corrected if far out, and the result will be better illuminated and better definition of the picture. Having satisfied yourself that the projection lens is right ; put a lantern slide on the screen and adjust the lens till it takes its proper position, then with the cards in position and a piece of card the same size as a slide — 3^4 inches square — pierced with a central hole, the centering of the lantern-slide attachment can be tested. The centering of this need not be so rigidly correct as the picture side, but if far wrong, or if any want of sharpness or evenness of illumination be noticeable, this method will at once show what is wrong, and indicate which way to move the several parts in order to effect a cure. LENSES. — THE CONDENSER. Before describing the particular forms and functions of the lenses used in the instrument, I will shortly state the properties belonging to lenses. All transparent substances have the /power of alternating the direction of rays of light passing through them, except where the substances in question consist of parallel plates, and when the rays fall exactly at right angles on the said parallel plates, in which case light passes through without -alternation of its direction. A lens is a piece of transparent material having one or both of its surfaces' curved, the curves being spherical in form. Each surface (when not flat) may be considered as part of the surface of a ball — large or small, according to the depth or convexity of the curve. On holding a "convex" lens — (one having its center thicker than its edge) — in direct sunlight, so that the light passing through it may fall on a sheet of paper, the light will be condensed into a space smaller than the lens; and in the case of a "concave" lens — (edge thicker than center) — the rays will diverge after passing the lens and occupy a space larger than the lens. In the first case the rays are bent towards the center; in the second, away from the center. The light, after it has passed through a convex lens, will be more intense than the original, because it covers less space; and less intense with the concave lens for the opposite