Wright: measurement of refractive indices 



105 



attained. This is well illustrated on plates of calcite which is 

 very strongly bu'efracting (co — e =0.172). 



In Table 1 are listed the computed extraordinary refractive 

 indices (e') for light waves entering a calcite plate under dif- 

 ferent angles of incidence. In columns 1 and 2 the values 

 are given for a plate cut normal to the principal axis; in col- 

 umns 3 and 4 ,the values for a plate cut parallel with the prin- 

 cipal axis; in columns 5, 6, and 7, the values for a cleavage 

 plate of calcite. Each of the above plates is considered to be 

 so placed that its principal section is parallel to the plane of 

 vibration of the incident waves. In columns 1, 3 and 5 are 

 listed the refractive indices for extraordinary waves trans- 

 mitted along directions contained in the plane normal to the 



•TABLE 1. 



principal section; it may be noted that the refractive index for 

 each of these columns remains constant for the different direc- 

 tions of transmission. In columns 2, 4, 6, and 7 the refractive 

 indices are given for light waves whose lines of propagation 

 are contained in the principal section. In these columns the 

 refractive indices vary noticeably with change in direction of 

 transmission. In column 6 the refractive indices e' are listed 

 for different angles of incidence i in the principal section of 

 the cleavage rhombohedron on the side toward the emergent 

 optic axis; in column 7 the refractive indices are given for light 

 waves incident also in the principal section but on the side 

 away from the optic axis. 



From this table it is evident that if a calcite cleavage flake 

 be immersed in a liquid of refractive index about 1.564, the 

 plate will appear to have approximately the same refractive 



