170 



BARUS— ADJUSTMENT FOR PLANE GRATING [April 24, 



riage C, measured on the scale ss, for the effective length of rod 

 ah = r between axis and axis, 



X^dx/r or {d/2r)2x, 



so that in either case A and x are proportional quantities. 



The whole spectrum is not however clearly in focus at one time, 

 though the focusing by aid of the rod hh is not difficult. For 

 extreme positions a pulley adjustment, operating on the ends of A 



Figs. 3, 4, 5. Diagrams. 



is a convenience, the cords running around the slide A A. In fact 

 if the slit is in focus when the eye-piece is at the center (^^o, 

 i = o) at a distance a from the grating, then for the fixed grating, 

 Fig- 4, 



a' ^ a 



