742 THE POPULAR SCIENCE MONTHLY. 



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 

 w r ould 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 prob- 

 ably existed in the sun. Furthermore, he found that the light from 

 the moon and from Venus gave the same system of dark lines. Fraun- 

 hofer saw in Venus-light the double D lines ; b also was seen double, 

 and the relative distance from D to HJ, 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 obser- 

 vations 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, how- 

 ever, 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 re- 

 solved. 



