The Moving picture world (April 1920-May 1920)

738 THE MOVING PICTURE WORLD May 1, 1920 Figure 3. Cosmograph Projector for Standard Film. A — motor; B — lamphouse; C — condenser casing; D — slot for slide carrier; E — stereo lens and its carrier; F — one of the gear casings; G — flywheel; H — shutter housing; I — reflector back of lamp. an oil well. All gears are also encased and packed in grease. Due to the construction of the mecha.iism objectives of any diameter or any focal length may be used. We have viewed the projection of a picture with the Cosmograph and must in justice say it compares very favorably in screen results with other projectors of its class. The Cosmograph is welcomed to the projection field by this department as offering a new and worthy addition to the famil.v of Portable Projectors. A Simple Chinese Puzzle Daniel Graney, New York City, is puzzled by the lens chart No. 2. Says that to him it is a Chinese puzzle. Well, brother Graney, it is very easily explained. Chart No. 2 is intended to tell you the correct distance to place the revolving shutter from the aperture in order that it will cut the light ray at its point of least diameter, which is the aerial image of the condenser. By setting the shutter at the aerial image it is posible to reduce the master blade of the shutter to its least possible width, and thus secure the maximum of illumination on the screen. First, let me explain exactly what the aerial image of the condenser is. The objective has what are termed "conjugate foci points." These points are respectively the object and image. On one side of the lens it is the distance from the optical center of the lens combination to the object, and on the other side it is the distance from the optical center of the lens to the image. The objective acts literally as a photographic lens. It photographs the film in the aperture, the image being formed at or on the screen. If there were no screen then it would be an "aerial image." In this instance the first distance is short and the second long. As you lengthen the distance from object to lens you automatically shorten the other distance. Photographs Front Surface. The objective also photographs the front surface of the condenser, but in that instance the object (condenser) is much farther away from the lens than is the case with the film; hence the other conjugate length (distance to screen) is immensely shortened, and will be just a few inches from the lens — sometimes, as with very short focal length lenses, even inside the lens barrel. But at that point there is an image of the condenser surface formed, which you may see quite clearly if you hold a sheet of black paper (black because the image is so brilliant that you could not see it clearly on white paper) at the point where it is formed. In lens chart No. 2 the distance of the aerial image from the aperture is indicated, and since the aerial image is the point at which the revolving shutter should be se*, it therefore is the position of the revolving shutter, with this proviso that, owing to the fact that brother Griffith based his calculations of disance on a simple, instead of a compound lens, all these distances are about one inch long. In other words, one inch should be subtracted therefrom. Lens chart No. 2 is of no large value, anyhow. It was merely given for the sake of completeness. I have wished sometimes that it had been omitted, because the correct position of the revolving shutter may be ascertained in a number of other ways, and quite a few seem unable to understand its workings. A Typical Example. Taking a typical example, say a five-inch E F objective, or projection lens as we then called it, with a nineteen inch distance from center of condenser combination to aperture, it is worked out thusly: First we find the five-i ch E F lens in the left hand column, it being the fifth from the top. We then find the nineteen inch column in "Distance from Condenser to Aperture," and where the two cross we find just nothing at all, but in the next column, which is for the 20-inch distance, we find 11.25 inches as the distance of shutter from aperture, from which one inch should be subtracted for error, leaving 10.25 as the correct distance where the lens is five inch E F, and the distance 20 inches. We also find that the size of the aerial image will be sixty-nine hundredths of an inch high by nine-tenths of an inch wide. This, too, was put in merely for the sake of completeness. It has no large value to the projectionist, except that he be advanced far enough that he can lay out his shutter blade width with the data, and few can do that as yet. Distance Is Little Altered. But all this is for twenty inches instead of nineteen. But if we go way back to the fifteen inch column we find the distance is altered but a little more than a quarter of an inch, so the difference as between the nineteen and twenty we may discard as negligible, insofar as concerns distance. If the distance were seventeen we would add the fifteen and twenty distances together and divide by two for the right distance, because fifteen is just half way between. It would, in fact, be very easy to compute the precise distance in the case under consideration by subtracting 11.25 from 11.6 and then adding one-sixth of the result to 11.25. True, the aerial image changes very much more with added or lessened distance, but that you can only use in calculating shutter blade width by geometry, which I think few will attempt. Flexible Calsomine Screen It has been suggested that a cloth screen iTiay be coated with calsomine and still remain sufficiently flexible to roll on a roller by mixing about a quarter of a cake of common yellow soap in each bucket of water used for the white fish glue sizing and the same amount in each bucket of water used for the calsomine. Of course, one would hardly use a full bucket of water for either the size or calsomine — certainly not for the latter, and a proportionately less amount ot soap should be used. Figure 4. Cosmograph Set for Stereo Projection. A — speed control; B — framing lever.