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OPTICAL SCIENCE IN CINEMATOGRAPHY 43
ing on the block of magnesium carbonate whereupon they are reflected in a perfectly erratic manner. In Fig. lb the rays leaving the source fall on the surface of the silvered mirror and are reflected in a perfectly correlated manner and in fact in such a way that they appear to emerge from point S1 back of the mirror and at the same distance from it as the light point is in front of it. S1 is an image of S formed by the mirror. An observer standing at position A in Fig. la will see the whole block of magnesium carbonate illuminated by the single point source of light so long as A is in front of the reflecting surface but an observer standing at a corresponding point A in Fig. lb will
Fig. la — (Left) Reflection from concave mirror close to source.
Fig. 2b — (Right) Reflection from concave mirror separated from the source by a distance slightly greater than the focal length.
see nothing of the surface of the mirror but will see an apparent second source of light at S1. No matter where the observer takes his position in Fig. la as long as he is in front of the magnesium carbonate block he will see it illuminated and his eye will receive illumination from it but in Fig. lb if the observer moves to position B he will not be able to see the reflected image of S and his eye will receive no light from the mirror. Within a certain region then it is obvious that the mirror contributes light but that region is definitely limited. Within the region in which it is effective it is exactly as if there were two sources of light, S and S\ instead of one. This is intended to be taken literally. If the mirror is taken away and a new source equal in intensity to S be placed at S1 the effect at point A will be identical with the effect prevailing when we had the one source and the mirror.
Suppose now we replace the piano mirror with a concave spherical mirror of any available radiums of curvature placing the mirror fairly close to the lamp in the manner indicated in Fig. 2a. Just as in the case of the piano mirror an observer standing in front of this set-up will see an image of the source of light formed in the mirror as long as he stays within a region which depends, under the conditions of the set-up, on the diameter of the mirror and which is included within position A and B in the figure. If the concave mirror