CHAPTER II. 



FKAUNHOFER, 1814. 



IN 1814 Fraunhofer extended Wollaston's work in both 

 directions, and he further introduced another improvement 

 by examining the emergent beam from the prisms directly 

 by means of a telescope, instead of allowing it to fall on a 

 screen, and then examining it with the naked eye, as Wol- 

 laston had done. In this way he got a magnified view of the 

 spectrum. In order to insure the utmost purity in some of his 

 experiments the slit was placed at a distance of 92 feet from 

 the prism. 



As may easily be imagined, by the aid thus afforded, he was 

 enabled to observe a much greater number of lines in the 

 solar spectrum than had been seen by Wollaston.. He con- 

 structed a map, of which Fig. 6 is a much reduced copy, in 

 which he recorded the positions of no less than 576 lines, 

 distinguishing the most prominent ones by letters of the 

 alphabet. From this time these dark lines have been called 

 Fraunhofer lines. 



He recorded also that the intensity of the light varied greatly 

 in different parts of the spectrum, being greatest in the yellow 

 and falling away rapidly on each side, as shown by the curved 

 line in the figure. 



Fraunhofer's work, in this direction, the importance of which 



