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824 HANDBOOK OF PROJECTION FOR
In other words the central six inches of them all is either spherical or very nearly so.
For a parallel (non-converging) beam the parabolic mirror is best; for a converging beam the elliptical mirror serves the purpose best. The spherical mirror is merely a corrected mirror. See figure 314A.
POSSIBLE MAXIMUM ANGLE.— The maximum angle of light which it would be possible for a reflector to pick up, or intercept would, in theory, be 180 degrees, which would necessitate the use of a reflector making a complete half circle around the light source. As a matter of fact, however, the angle which could be used in practice would be somewhat less than this, because of the fact that little or no light actually leaves the crater in a direction at full right angles thereto.
Exhaustive tests have been made by Dr. Simon Henry and Dr. Henry Phelps Gage, and by the late Doctor Kellner, of the Bausch and Lomb Optical company, which same have proven that the maximum light angle any reflector can pick up and efficiently utilize in the form of a solid beam, is 120 degrees, hence the mirror shown in figure 310A may be said to be operating at the entire possible maximum of efficiency.
ABERRATIONS.; — In reflector arc lamp optical systems two distinct types of reflector are used, Viz : the parabolic and the spherical. The spherical mirror is one the curve of which is the like circumference of a circle. In other words it is the section of the surface of a true sphere, the focal point of which is the center of the sphere. This is illustrated in figure 313A, in which D is the reflector, or mirror, A the focal point (center of curvature) of the mirror. From A to any point on the surface of the mirror is the measurement of the focal length of the mirror.
With such a mirror, were the light source placed at A (This would only be precisely true were the light source a "point," but this item is neglected, as we only seek to have you understand the principle), it would be exactly plane to every point on the mirror surface, under which condition there would be no spherical aberration at all, but the image of the light source would be focused directly upon the light source itself.
This would, of course be impossible for projection purposes, since the film must be illuminated by the light reflected from the mirror, hence the beam must not only be extended beyond point A, but also its cross section at the point it meets the