Page:Astronomy for Everybody.djvu/98

ance affects the optic nerve of the eye so as to produce a sensation of light and enable us to see bodies.

It is now know that radiance consists of something in the nature of waves in an ethereal medium which fills all space, even to the most distant star. These waves are exceedingly short. To form an idea of their length we must measure by the micron, which is one thousandth of a millimetre. Those which produce the sensation of light on the optic nerve mostly range between four and seven tenths of a micron.
Fig. 14.—Wave Length of Light. This allows between forty and eighty thousand waves to the inch. We represent these waves by the little wave line in the figure. The distance between the dotted lines is the wave lengths. The peculiar feature of the radiance emitted by the sun, or any other body that is not transparent, is that it is not all of the same wave length, but of a very wide range of wave lengths all mixed together. We must imagine that between the rays which we represent in the figure there are an infinity of others, all varying in their wave lengths. In this respect radiance is like the waves of the ocean, which range in length from several hundred yards to a few inches, all piled upon each other.

When the radiance passes through a glass prism it is refracted from its course. Different wave lengths are refracted differently, but waves of the same length are always refracted by the same amount. This is shown by the familiar experiment of forming a spectrum of the