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46 CINEMATOGRAPHIC ANNUAL
marginal zone of the mirror is too great so that they cross ahead of the light source and give rise after they cross to a diverging beam. From the standpoint of geometrical optics, the curvature of the spherical mirror is too great as we go from the center toward the margin. What is needed in order to cause the marginal rays to become parallel with the axial or central rays is a mirror whose curvature decreases from the center toward the margin. Exactly the required decrease of curvature is provided by the parabola. Any ray emanating from a point source of light placed in the focus of a parabolic mirror is reflected by the mirror parallel to its axis as shown in Fig. 3b. In other words, a point source of light placed in the focus of such a mirror gives rise to an absolutely parallel beam of light.
Lest the use of the expression "parallel beam of light" in one of two places lead anyone to erroneous conclusions; it should be said right away that such a thing as a parallel beam of light cannot exist. In fact, the only possible way in which a parallel beam of light could be obtained would be with the use of a point source of light which is an impossibility. In order for a source of light to give any light, whatever, it must have finite dimensions. If the source of light has finite dimensions, the minimum angle of divergence of the beam of light can never be less than the angle subtended by the source of light at the mirror or lens employed with it.
In addition to mirror spots condenser spots are also employed. The theory involved is identical. If a condenser spot light is adjusted to give the smallest and brightest spot of light, an observer taking his position in the center of this spot and looking at the condenser lens will see that it is entirely filled with light or, in other words, the purpose of a condenser is to replace the original source of light by another source equally bright but much larger in diameter and, therefore, capable of giving us a correspondingly increased amount of illumination. The other lighting units have practically nothing of interest from a standpoint of optics involved in them, therefore, we will pay no further attention to them. For one, who may desire to read further in the subject of the behavior of mirrors and who is able to read German, two excellent and interesting little books are mentioned at the end of this article. The second is a translation into German of the work of Mangin and Tschikolew, French and Russian military officers respectively who were interested in the performance of concave mirrors for military signal purposes.
We now come to that most interesting of optical implements, the photographic lens. When photography was born, the only lenses available were single achromatic meniscus lenses, very low in speed and very limited as to the size of the field of view they would cover with satisfactory definition. These slow lenses combined with the very slow emulsions available at the time made photography almost hopeless.
In 1841 Joseph Petzval accomplished at one stroke an enormous advance in the speed of lenses. Within a few years, other people improved on his design in point of speed and succeeded in obtaining