International projectionist (Jan-Dec 1935)

14 REVIEW NUMBER May 1935 ably won't believe this, but we'll prove it sometime. Taking leave of plane waves for a while, let us consider diverging waves. You will note that the action in Fig. 6 is exactly the same. The belly flattens out, the front straightens out; then the ends get out first and the front begins to converge, but comes to a point much further out. These two points are called the conjugate focii of the lens; "jugate" means joined; "con" means together; the "conjugate focii" are the two points joined together or related in this particular lens, and a point light source placed although it will cut down the light. Another difficulty is chromatic aberration (Fig. 10). We said at the begin(((ft-A ning that a pencil of light (represented by a line) was made up of millions of rays of light. If the light be white, it is made up of rays of many different wave-lengths — thousands of them, perhaps. Now, the effect of glass upon the speed of these rays is far different for each individual wave-length, so that if you had a lens corrected for spherical aberration, all the rays still would not come to a point: the blue would focus at, say, b and the red at r, and all the others in between. •A Horizontal ray ti ?71S No. 11 Object ^^^ | Jxis of lens ^""""-^J? ^\Tocus No. 12 Image D Chromatic Aberration Fortunately, chromatic aberration is comparatively easy to correct. Assuming that this lens is made of crown glass, a lens of flint glass of the same size and shape would spread the blues and reds at either place" would be focused at the -uch further although the Jo-l poim other. (Note that I said point source.) If we move this point source closer to the lens( Fig. 7) the emerging front is curved less and less until when the light is at the real focus of the lens, the emerging wave-front is plane. Spherical Aberration When we move the point source in further to a point within the principal focus of the lens (Fig. 8) the wavefront does not get a chance to straighten in general would not be much different. A negative lens of crown glass would spread the light in the opposite direction, with the reds closest to the lens and the blues further away. And, a lens of flint glass would also spread them in the opposite direction, only still further apart. So, to correct for chromatic aberration (Fig. 10A) we take advantage of these properties and combine a crown glass No. 15 glass positive lens with a flint out, the ends never do catch up, and it negative lens, which brings the blues and emerges as a diverging wave. This is all reds together but makes a combination very simple — much too simple, in fact, 0f a little longer focus. Here again describe briefly the point-to-point method for in practice nature hands out a lot only two colors can be corrected for with 0f locating an image from an object, of jokes and jolts to such things as two glasses; the others still are out, and Just two simple rules are used, and while that in Fig. 12) and make our measurements from the center of the lens. While we are considering this drawing, I might preconceived notions and the like. We will spend a few minutes considering some of these "jokers." First we shall consider spherical aberration (Fig. 9). The very thing that makes a lens change a wave-front (the varying thickness of glass) also causes the rays at different distances from the center of the lens to focus at different points — with the outer rays focusing closer, and the inner rays focusing farther away from the lens. This condition is corrected by various methods of grinding and by combinations of lenses. We have an example of partial correction in Cinephor condensers, wherein one surface is parabolic and the other spherical. One point frequently overlooked is that even in the very best photographic lenses spherical aberration cannot be wholly corrected. The outer rays may be made to meet the inner rays, but the intermediate rays will not meet at the same point. The larger the diameter of the lens, of course, the greater the difficulty of correction. Hence, in a camera the smaller the diaphragm, the sharper the picture; in a projector, a diaphragm will sometimes improve an inferior projection lens, it is customary to correct for the two brightest colors. Some high-grade photographic lenses have three glasses and correct for three colors. Curvature of field must be corrected for (Fig. 11) to bring the outer edges of the picture in focus on a flat screen when the center is in focus. There are also the problems of astigmatism, coma, distortion and others which we shall not consider at this time. It is frequently asked from what point does one measure to get the focal length of a lens — from what part of a condenser lens and from where on the barrel of a projection lens. The answer is, generally: from somewhere inside the glass of a condenser and from somewhere between the two combinations in a projection lens. When dealing with very thin lenses we may consider the lens as a line (like the problem can be very complicated, in this case the application is easy: 1. Horizontal rays always refract to the focus. 2. Central rays pass through without deviation. The horizontal rays from the head of the arrow (Fig. 12) are refracted through the focus, and the central ray passes straight through the center; where the two meet is located the image of the starting point. Here is a diagrammatic representation (Fig. 13) of a compound microscope which traces the paths through the objective lens and eye-piece lenses and shows the lenses as represented by straight lines, the particular shape of the glass making no difference. In fact, lenses are designed from such drawings, and when the separation and focal points are determined so as to get the results No. 9 P M