OPTICAL CHARACTER OP BIREPRACTING MINERALS. 
75 
of propagation are not prescribed, as is the case in anistropic crystals. A 
light- wave, vibrating in any given azimuth t in an isotropic medium, will 
continue to vibrate in that azimuth, as there is no force active in the 
medium which tends to alter its plane of vibration. Similarly a plane 
polarized light-wave in passing from one isotropic medium to a second en- 
counters no forces, due to the structure of the adjacent media, which tend 
to divert the line of vibration from its original plane. Thus the line of 
vibration of a plane polarized light-wave in passing from air into a glass 
plate suffers change in direction, but it is still contained in the original 
plane of vibration, as there is no force present in the system which pre- 
scribes that it shall vibrate in another plane ; the direction of propagation 
of the wave (and with it the wave-front) is, however, changed on entering 
the glass plate ; the line of vibration of the refracted wave must be contained, 
therefore, in both the original plane of vibration and the wave-front of the 
FIG. 47. 
FIG. 48. 
refracted wave; its direction accordingly is definitely fixed by the line of 
intersection of these two planes. In Fig. 47 let MM' be the boundary sur- 
face between air and the glass plate MM'LL'\ let a wave AO be incident at 
and let the plane EON represent the wave-front of the incident beam ; let 
also EO be the line of vibration and AOE the plane of vibration; similarly 
let OB and FON be the direction of propagation and the wave-front re- 
spectively of the refracted ray. Then the azimuth t (angle O.V) of the 
line of vibration of the incident wave is definitely fixed by stating the angle 
which it includes with the line normal to the plane of incidence. To pre- 
sent the same phenomena from another viewpoint, let EOX in Fig. 48 be 
the wave-front of the incident wave and FOX the wave-front of the re- 
fracted wave ; let EO be a line of vibration in the incident wave-front EON. 
The problem of determining the line of vibration in the refracted wave- 
front consists simply in finding the line of intersection of the plane FON 
(the wave-front of the refracted ray) with the plane of vibration passing 
