ipog.] 



SIMILAR TO ROWLAND'S METHOD. 



169 



is preferably made with a narrow beam of sunl'-ght, though lamp- 

 light suffices in the dark. Thereafter let the slit be focused with 

 the eye-piece on the right marking the position of the slit; next 

 focus the slit for the eye-piece on the left ; then place the slit mid- 

 way between these positions and now focus by slowly rotating the 

 grating. The slit will then be found in focus for both positions 



\N"\ 



O ^l:f\ 



Fig. 2. Elevation of the grating {g) and the eyepiece {E) standards. 



and the grating which acts as a concave lens counteracting L will 

 be symmetrical with respect to both positions. 



Let the grating be thus adjusted when fixed normally to the slide 

 B or parallel to A. Then for the first order of the spectra the 

 wave-length A = d sin 6, where d is the grating space and 9 the 

 angle of diffraction. The angle of incidence i is zero. 



Again let the grating, adjusted for symmetry, be free to rotate 

 with the rod ab. Then 9 is zero and X = d sin i. 



In both cases however if 2.r be the distance apart of the car- 



