( 149 ) 
If we substitute this in one of the equations (30), (31) and (32), 
we find: 
Ne in ae ny). 
8 
e s . . 
The expressions /A-+ mu 4 nv and — both being the cosine of 
v 
the angle between the ray and the normal to the wave-front, so that: 
nn tiny =, or get jae cae Olen een (OD 
Uv 
Vv 
we find A = —; consequently: 
v 
Bb), a ec 4G. et ee ee 
v 
GS = We va, ed tan ere ee 
v 
ea 1b), Ge ede 
Vv 
or 
€=—sin (1. B). (65) 
OND atx 6 a7 ts 
In the same way we deduce: 
? ain 
as Higher oe en ee 
v 
2 ie 
B= wf — kde en see ed NN 
Uv 
Q nk 
Gehe 7 ae on Se 
or 
—p=Ssin (1 2). le es 
10 
Proceedings Royal Acad. Amsterdam, Vol. IV. 
