F. H. Richardson's bluebook of projection (1935)

114 RICHARDSON'S BLUEBOOK OF PROJECTION are used for glass condensers has considerable value because it serves to "break up" the image of the light source, which never has even luminosity over its entire area. Spherical aberration has some value in preventing the image from being projected to the screen in the form of uneven illumination. Lens Cannot Focus to a Point (55) Unless the light source be infinitely small — a pinpoint, in fact — a lens cannot possibly focus light coming from it to a point. A lens focuses light rays to form an image of a light source or object and the image may be either smaller, larger or of equal size with the light source or object. (56) Stating it another way, rays are sent forth from every pinpoint of a light source, or from an object from which light is reflected. Rays from this pinpoint and from every other pinpoint of the light source or object are picked up by every section of a lens within their view which usually means its entire area. These rays are refracted by the lens and sent forward to a screen placed exactly at the other conjugate foci point of the lens, where they are refocused to a point which may, however, be larger than the point from which they come. Millions of these refocused points, each in its exactly appointed place, constitute an image of the object or light source from whence the rays emanate. The Projection Lens (57) The projection lens is known as a compound lens, which is a lens having more than one element. It has four lenses in fact, though two of them are usually cemented together with Canadian balsam. At the rear of the lens (end next the film) are two lenses, usually separated slightly by a spacing ring. At the other end of the lens barrel is the "front element/' consisting of the two lenses cemented together. At a superficial glance they will appear to be one thick lens. (58) Sometimes the Canadian balsam, with which the lenses are cemented together, will melt, producing a