Motion Picture News (Mar-Apr 1923)

1204 Motion Picture News Figure 9 Figure 10 i^JIlnfJl'AZt li*htJrom an irregular surface. Fig. 10. Illustrating the relationship between lenses and prisms If we consider a lens as consisting of innumerable small prisms built up around a common center this relationship will beome apparent that the lens is a continuation of an infinite number of prisms, the flat surfaces of which are too small for the eye to detect. This infinite number of surfaces, or points, we find, ranges itself into the segment of a circle. This refraction of rays emanating from a point back to a point again is termed a " point of focus." If we now take two luminous points at the same distance from the lens but separated a short distance from one another, as in Figure No. 12, we will find if we have a screen for the rays to fall upon, that the two points will be reproduced side by side in exact miniature (Continued on page 1208) (Continued from page 1202) them, for the light, coming from so large an area covered by the tubes, is already sufficiently diffused. When we produced the spectrum by passing a ray of light through a glass prism we found that the beam of light was bent or turned to one side by the glass ; that is, the light was refracted. This refraction only takes place at the point of entrance between two mediums of different density. After being refracted at the surface the light continues to travel through the second medium in a straight line from the point of entry to the point where it emerges on the other side where a second refraction takes place, light again continuing to travel in a straight line. This angle of refraction varies according to the density of the medium in its relation to light and is always the same in the same medium, thus different kinds of glass and all transparent crystals and liquids have different angles of refraction. This angle of refraction is called the index of refraction. These indexes of refraction have been measured by mathema ticians who make calculations for manufacturers of lens and predict all of its properties before one has been made. Such calculations are, however, far beyond the scope or needs of any ordinary photographer. In Figure No. 10 we have a point from which emanates rays of light. Suppose we take a number of prisms with varying angles as illustrated in the diagram, the angle of Fig 11 This is the same as Figure 10 with the proper curved surfaces substituted for the angular surface of the joined group of prisms Fig. 12 // ive take two luminous points, A and B, we find that the lens will form images of these two points as a and b. The point A being on the principal axis of the lens its image will be formed at a, also on the principal axis any motion of B will cause a diametrically opposite motion in b each being such that each ray which passes through each prism is refracted to the point so that each of these rays is again collected at this point. Let us now examine the line oi prisms which we have thus placed. The central prisms have sides which are nearly parallel, which progress outward from the center, the angle increases until the two faces come together. We will now replace the line of prisms with a lens covering practically the same range as the prisms as in Figure No. 11. We find that the lens also gathers all of the rays as the prisms did and refracts them again to the same point, so that we can consider the lens as a number of prisms rounded off into a single piece, or speaking still more exactly, Fig. A3 Let us take the points A and B in these two diagrams. In both the upper and lower diagrams the image of A will form in the plane a and that of B will form in the plane b. It is in these two planes that the sensitive surfaces should lie to render sharply the images of A or B as the case may be We desire to receive both of these images, however, on the plate at once and utilize the two following means for obtaining the result. First we compromise between the two planes a and b and place our plate in the plane " C." We do this becaue the circle of confusion at C, is common to both and is the smallest mean between the planes a and b. This compromise prepares us for better results in our 2d procedure. This consists of placing a diaphragm close to the lens. This diaphragm is a piece of black cardboard with a smooth, round hole in it and its function is to diminish the angle on the rays of light that represent the extremes of the cones of light which form the images a and b. This has the desired effect of reducing the size of the circles of confusion at C to an inappreciable size. This size depends on the distance between A and B and on the size of the hole in the diaphragm. A circle not greater than 1/100 inch is permissible in stills but for the cinema film one of 1/400 inch is about the limit of size This Machine with this Mark > — is a low-priced cashier that NEVER MAKES A MISTAKE Write for free demonstration and easy-to-pay low prices AUTOMATIC TICKET REGISTER CORPORATION Largest in the World 1782 Broadway New York City More tickets are printed by us than by any other firm in the world because GOLD SEAL tickets are made and priced right. T IT COMPLETE INFORMATION on all phases of Theatre Construction, Equipment and Remodelling will be contained in a Special Issue of the Motion Picture News This issue will be published in the near future.