82 



proportion. It can be shown that the effective resolving power 

 depends on the material of the prism, which must be as highly 

 dispersive as possible, and on the size, or number, of the prisms 

 employed; and by increasing these it has been found possible 

 to "resolve" double lines thirty or forty times as near together 

 as are the sodium lines. It will be convenient to take the 

 mea.5ure of the resolving power when just sufficient to separate 

 the sodium lines as i,ooo. Then the limit of resolving power of 

 prism spectroscopes may be said not much to exceed 40,000.* 



This value of resolving power is found in practice to obtain 

 under average conditions. Theoretically there is no limit save 

 that imposed by the optical conditions to be fulfilled — and es ■ 

 pecially by the difficulty in obtaining large masses of the re- 

 fracting material of sufficient homogeneity and high dispersive 

 power. It is very likely that this limit has not yet been reached. 



Meanwhile another device for analysing light into its com- 

 ponent parts has been found by Fratmhofer t, which at pres- 

 ent has practically superseded the prism ; namely, the diffraction 

 grating. Fraunhofer's original grating consisted of a number 

 of fine equidistant wires, but he afterwards made them by rul- 

 ing fine lines on a glass plate covered with gold-leaf and re- 

 moving the alternate strips. They are now made by ruling upon 

 a glass or a metal surface fine equidistant lines with a diamond 

 point. 



The separation of light into its elements by a grating depends 

 on its action on the constituent light-waves. 



♦Lord Raylf igh has obtained results from prism of carbon disulphide which promiiet a 

 • uch higher resolvins: power. 



+1821. 



