We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
To parallel the light rays with these two lenses the arc should theoretically be 6^ inches from the lens. But this is not best, for at the closer position more than four times as much light at the aperture results. If the rays could be paralleled by the first lens, the arc lens, then the converging lens might be any distance away. Because the arc lens cannot do this, the second lens, the converging lens, is brought up close to the arc lens in order to catch as large a portion of the rays as possible and concentrate them on the picture aperture.
Authorities on lenses have heretofore recommended that these converging light rays cross in the center of the projecting lens, but their conclusions have been based upon the old lantern slide assembly which had the pictures just in front of the converging lens, and did not employ a shutter at all. Their recommendations are not, therefore, wholly applicable, nor is the arrangement proposed for the lantern the best for motion picture projecting machines.
It might be nearer right if we could get lenses made so that the shutter could cut across the rays at the narrowest part, that is, at the diaphragm location in the lens. But this is not practical, because, among other things, a variety of focal lengths of projecting lenses are required for different projection distances, or "throw" as the operator usually terms it. The preferable arrangement is, therefore, to put the shutter in front of the lens, and then to have the rays of light cross at that point.
The principal reason for having the shutter at the narrowest part of the projected light-beam is that the obscuring blade may be as narrow as possible, so that when the other two blades, the flicker blades, are added they may each approximate the width of the obscuring blade and yet give a fifty-fifty ratio of light to darkness, which is the ideal arrangement for a flickerless picture today.
To readily determine where the light beam is narrowest is not alwasy easy, for the light cone is not readily discernible. If, however, a card is passed vertically through the light just in front of the lens, a shadow will cross the screen, say, from top to bottom ; with the card farther out the shadow will cross from bottom to top ; but at a median position the passing of the card causes a shadow to cover the whole screen at once. That is the proper location for the shutter. One should remember, however, that a shutter that is just right, that is, most efiicient, at this point will show halation or streamers, both above and below the letters of a title, if the arc is moved very much either toward or away from the condensers.
There are other factors also which have a bearing on the location of the various elements of the projection system. For example, a longer projection for the same size picture requires a projection lens of longer back focus. This again requires a longer cone of light from the converging lens. This lens must, therefore, be changed for one having less curvature. But when this is done the light spot on the aperture is too large, and the lamp house must be moved back. This draws the apex of the light cone back within the projecting lens barrel, so that another change is required. By repeated trials one would