Page:Elementary Text-book of Physics (Anthony, 1897).djvu/458

444 as the source of light. $$C$$ is an achromatic lens, called a collimating lens, so placed that $$S$$ is in its principal focus. The waves emerging from it will then be plane. These will be deviated by the prism, and the waves representing the different colors will be separated, so that after passing through the second lens these different colors will each give a separate image. These images may be received upon a screen, or observed by means of an eye-piece. Sometimes a series of prisms is used to cause a wider separation of the different images.

If the images at $$F$$ be received on a sensitive photographic plate, it will be found that the image extends far beyond the visible spectrum in the direction of greater refrangibility, and a thermopile or bolometer will show that it also extends a long distance in the opposite direction beyond the visible red. The solar radiations, therefore, do not all have the power of exciting vision. Much the larger part of the solar beam manifests its existence only by other effects. It will be shown that, physically, the various constituents into which white light is separated by the prism, differ essentially only in wave length.