igii.] 



WITH REFLECTING GRATING. 



133 



uring the latter it was necessary to count the fringes between the 

 C and D lines and divide their angular distance apart by these num- 

 bers. As c cannot be measured, its successive increments Ae from 

 the first position are given. These are presently to be associated 

 with the corresponding increments of dn/dO. 



TABLE I. 



Values of dO/dn. etc. t' = 53° 15'. 7? = 200 X 10"" cm. 



8. Equations. — In Fig. 5, L and L' represent a pair of corre- 

 sponding white rays, reflected into R and R' and diffracted into D 

 and D' at angles i and 6. respectively. The half gratings g and g' 

 are separated along the crack S\ and g' is movable parallel to itself 

 by a micrometer screw normal to g'. Let the normal distance apart 

 of the gratings be c. The incident rays L, L' strike the originally 

 coplanar grating at points A' rulings apart, or XD cm. apart, if D 

 is the grating space. In the separated grating let these points be at 

 a distance c apart. Let d be the incident wave front and /; the corre- 

 sponding dift'racted wave front and call the angle between r and d. y. 



When there is reinforcement the path dift"erence of the rays L 

 and L' from the incident (d ) to the dift'racted (/;) wave front, may 

 be written ;;A ^= b — a. 



where b and a are the distances of /; and d from the points of inci- 

 dence of L and L' on the grating g and g' respectively. If finally / 



