Page:Astronomy for Everybody.djvu/99

sun with a triangular prism. Arranging the light to be thrown on a screen, we see red light at the bottom then yellow above it, then in succession, green, blue, and violet.
Fig. 15.—Arrangement of the Colours of the Spectrum, with the Dark Lines A, B, C, D, etc., of the Spectrum. This arrangement of colours on a surface is called a spectrum. The colour of the light in the spectrum depends on the wave length. If the wave length is greater than about seventy-five one-hundredths of a micron, that is, one forty-four-thousandth of an inch, the eye does not see it, and, for us, it passes simply as heat. From this length to one fifty-thousandth it looks red, when a little shorter it looks scarlet, then yellow, and so on. Shorter than forty-three one-hundredths of a micron it is difficult to see it at all. But the violet light affects the photographic plate even more strongly than the light which looks brightest to the eye. The light which is most easily photographed is the blue and violet, and as we go toward the red the photographic effect diminishes.

All bodies emit radiance, but, at ordinary temperatures, the wave lengths of this radiance are too long to be visible to the eye. Not until we heat a body red hot does it emit radiance of wave length short enough to