Page:Astronomy for Everybody.djvu/101

a gas which is cooler than the body, the latter will cull out and absorb from the light those wave lengths which it would emit if it were itself incandescent. The result is that the spectrum from the solid body will be seen crossed by certain dark lines, depending on the nature of the gas through which the light has passed. Thus, if we observe an electric light through a prism in its immediate neighbourhood, the spectrum will be unbroken from one end to the other. But if the light is at a great distance, we shall see it crossed by a great number of dark lines. These lines are produced by the air through which the light has passed culling out the light which has certain wave lengths. It is of interest that the aqueous vapour in the air is the most powerful agent in this, and culls out great groups of lines, by which its presence in the air can be immediately detected. The darkest of the lines found in the spectrum of the sun are designated by the letters A, B, C, etc., as shown in the preceding figure.

We may describe the spectroscope in the most comprehensive way by saying that it is an instrument for studying the spectra of bodies, whether in the heavens or on the earth.

The studies of the heavenly bodies with the spectroscope have two objects. One is to determine the nature of the bodies; the other their motions to or from us. The possibility of the latter is one of the most wonderful achievements of modern science. If a star is coming toward us, the wave length of the light which it emits is slightly shorter in consequence of the motion; if it is