58 Reflected-Dif varied and Diffracted-Refiected Bays. 



constant, and it makes no difference where the mirror M may 

 be placed, i. e. how great the absolute vertical height of the 

 illumination h may be. 



In case of this method (parallel light impinging on the 

 slit) the illumination at each point of the image is received 

 from but a single point (nearly) of the mirror, whereas if the 

 light falling on the slit is convergent, the whole vertical 

 extent of the mirror illumination contributes to each point of 

 the image in the ocular. Hence in the latter case the fringes 

 are only sharp when M and the grating are rigorously 

 parallel, and they soon become blurred when this is in- 

 creasingly less true. The same observation also accounts 

 for the greater difficulty in adjustment when lamp-light is 

 used. In any case, equation (25) furnishes N/S. N may be 

 obtained with an ocular micrometer. The angle 7 may also 

 be found by actually measuring the inclination to the vertical, 

 /3, of the fringes in the ocular. Here if the height of image 

 s in the ocular corresponds to the vertical length of slit S, 



H-K-0 ™ 



while 



Nftdd' 

 P ~ s dn> 



where dO'/dn is given by equation (17). Hence s may be 

 eliminated and 



JY 



If, now, we further eliminate JS r /S in equation (25) by 

 equation (27), we have finally 



PK(J t + 2(l)-f.) 



7 2f c f t (cos 0' — cos i) dd'jdn ' 



so that 7 is given in terms of /3, the observed inclination of 

 fringes in the ocular. To measure j3 the ocular must be 

 revolvable on its axis so that the cross-hairs may be brought 

 into coincidence with the fringes, and the angles found. To 

 measure iV, the I) lines, if in focus, may often be used for 

 reference in place of vertical cross-hairs, as they remain 

 vertical. 



