1] APPLICATION AND MEASUREMENT OF LIGHT 155 



on a prism of bisulphide of carbon,* which gives very great 

 dispersion of the rays. 



In defining the regions of the solar spectrum which are 

 employed in any study, it is usual to make reference to the 

 dark absorption bands (FRAUEXHOFER'S lines) which cross 

 the solar spectrum. The largest of these are lettered, begin- 

 ning with A in the visible red and ending with H in the 

 visible violet. At other times it may be more convenient to 

 define any part of the spectrum by means of the extreme 

 wave lengths between which it lies. Lithographs showing the 

 spectral colors and the wave lengths corresponding thereto 

 are given in encyclopaedias and most of the text-books on 

 physics. A crude attempt is made to show the relation be- 

 tween color and wave length in Fig. 40. The wave lengths at 



Indigo 



Violet 



FIG. 40. Diagram of the solar spectrum showing the main absorption bauds and the 

 range of the various spectral colors. The numbers are wave lengths in hundred- 

 thousandths of a millimeter. (From REIXKE, '84.) 



9 



the different absorption bands are given more exactly (in thou- 

 sandths of a millimeter, = /-i) in the following table, and also 

 the number of waves per second in 10 12 ths. 



TABLE XVI 



* The bisulphide prism may be made as follows : Upon a thick glass plate 

 three rectangular pieces of glass of equal size are placed perpendicularly, so as 



