580. Dispersion

Dispersion. But glass possesses at the same time another property which is a distinct disadvantage to the lens maker. A ray of sunlight falling on a prism of glass would be bent according to the laws of refraction and proceed as in Fig. 2, providing that the prism had no other effect on it than to bend it. But if we place a white screen in its path after it emerges from the prism, we find that instead of a disc of white light, as we would expect we have a band of color, Fig. 3. This shows us immediately two things; first - that a ray of white light is composed of several rays of different colors, red, orange, yellow, green blue, indigo and violet, and second - that the different colored rays are differently refracted, and consequently separated, in passing through the prism. This separation, spreading apart, is termed dispersion and is caused to a different degree by different kinds of glass.

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Fig. 2.

581. Visual And Actinic Rays

Visual And Actinic Rays. The band of colors is called the spectrum. The different colored rays have different luminosity, the yellow being the brightest. But this is not the only difference which concerns us. If the spectrum falls on a strip of photographic printing out paper, we find that the paper soon begins to darken at and beyond the violet, but that no darkening, even after quite a while, is perceptible beyond the blue toward the yellow. This shows that the blue and violet rays have the strongest action on the sensitive silver salts. Thus we distinguish between I. - the luminous or visual rays which form the visible photographic image, and II., the actinic or chemical rays, which affect the sensitive surface on which the image is received. 582. Chromatic Aberration. - If we have two prisms identical in shape, made of the same kind of glass and placed base to base, a pair of rays entering those prisms as shown in Fig. 4, will be similarly affected. A lens made of a single piece of glass (a "simple" lens) and corresponding in shape to the combined prisms, as also shown in Fig. 4, would simply be a modification of those prisms (a lens being in effect a combination of an infinite number of prisms of different angle) and would produce a similar effect. For photographic purposes such a lens would be of very little value. Let us assume that the two rays have proceeded from a point on some object. Instead of meeting as one pair, at one point, to form one image, they are divided up into several pairs meeting at different points and forming several images, each pair of rays of the same color forming an image at their point of meeting. The most luminous image is the one formed by the yellow rays. Focusing this image as sharply as possible and exposing a plate, we do not get a sharp image in the negative. The reason is apparent. The blue and violet rays have the strongest action on the plate. These rays have their meeting point, their focus, nearer to the lens than the yellow rays and are consequently out of focus, forming only a blurred image when they reach the plate. The resulting image in the negative is therefore blurred; the yellow rays which would have formed a sharp image, having had no perceptible effect. The only remedy is to bring the plate closer to the lens where the actinic rays form a sharp image. This fault in a lens of bringing the actinic and the visual rays to a focus at different distances (in different planes) is called chromatic aberration - color aberration - and no lens formed of a single piece of glass can be free from it.

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Fig. 3.

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Fig. 4.