Page:Popular Science Monthly Volume 6.djvu/762

742 in sunlight that which takes the shape of lines when passed through a narrow opening. The white image from such an opening would of course be a fine white line. If passed through a prism the white line would become a series of colored lines, each point or ray of which would be an image of the slit. If the slit is changed to a cross, then there are crosses in the spectrum. The dark lines, of course, are lines in which light is absent, for darkness is absence of light. There are then certain missing rays in the sunlight that come out in the spectrum as lines of vacancy or breaks in its continuity, and Fraunhofer's lines are all of this kind.

Now, Fraunhofer made the important discovery that the lines of sunlight did not vary when examined at different times. His 590 lines were there, in their exact places, at all times of the day, and at all seasons of the year; the cause was therefore probably not in the earth's atmosphere; did not pertain to the earth, and therefore probably existed in the sun. Furthermore, he found that the light from the moon and from Venus gave the same system of dark lines. Fraunhofer saw in Venus-light the double D lines; b also was seen double, and the relative distance from D to E, and from E to F, was the same in the Venus as in the solar spectrum. As this light is reflected from the sun, Fraunhofer was confirmed in the conclusion that these lines are of solar origin.

But he went still further. He made careful and extensive observations of the spectra from the fixed stars, and made the striking discovery that they give groupings of dark lines, which differ from those of the sun and from each other. Some of the stellar lines, however, he showed to be identical with those of the sun. Among the lines of the bright star Procyon he recognized the solar line D; and in those of Capella and Betelgueux he found both D and b. Fraunhofer made also the important observation that the bright-yellow line characteristic of the spectrum of sodium exactly coincided with the double dark solar line D. But he could not take a step toward explaining the connection. It was impossible for him to know in what way special rays were cut out or absorbed in the sun and stars, so as to give only darkness, but he got clearly before him the great problem which it is the glory of spectrum analysis to have since resolved.