to the Wave Theory of Light. 



37 



Through the point S, where the incident ray $'#, prolonged, 

 cuts the circular section of the sphere, draw SH perpendicular to 

 the face of the crystal, or to FA ; and let SR, produced, cut the 

 circle again in the point s. Then Os is the direction of the ray 

 given by ordinary reflection at the first surface of the crystal. 



Fig. 15. 



Produce the right line Sits both ways, to cut the surface of re- 

 fraction in the points P, M, behind the crystal, and in the points 

 p, m, before it ; and conceive planes to touch the surface of re- 

 fraction at the points P, Jf, p, m. Suppose also that perpendi- 

 culars OP 7 , OJT, Op', Om f , are let fall from upon these tangent 

 planes, and that they intersect the planes in the points P', M' , 

 p', m', respectively. 



Then from the preceding observations (33, 34, 35), it is 

 manifest that OP', OJf' , are the directions of the rays into 

 which S f is divided by refraction ; that each of these refracted 

 rays, on arriving at the second surface of the crystal, is divided 

 by internal reflection into two rays parallel to Op', Om' ; and that 

 each of the four reflected rays, on arriving at the first surface, is 

 again divided by reflection into two rays parallel to OP', OJ/', 



