Telescopes and Spectroscopes for Lines of Finite Width. 333 



the case of fig. 3, be closely represented by an empirical 

 hyperbola (dotted curve) whose equation is 



a f(w) JK ' w+a/ v J 



whence 



n=Lv+-^- (23) 



7 IV + CC v 



Fig. 7. 



2.0 3 



The angular width, w, of the line, since this is produced by 

 the dispersion of the spectroscopic train, is 



w = DA\=gAX, (24) 



o 1/4 - x 



= »(r^+^\); • • • < 25 > 



and therefore for the spectroscope resolution 



fiW-°. 



and 



a formula very similar in form to that derived for the purity 

 P in the case of a wide slit and monochromatic radiations. 

 Phil. Mag. S, 5. Vol. 43. No. 264. May 1897. 2 D 



