Rays, i?i Crystals mth one and tv;oAxes of DoubleRef taction. 1 39 



2. If the edge qfthejprism is parallel to the axis B, we have 

 always s = z" , and consequently n. 



w'« = A and t;"^ = A + B sin^ s". ^ 



The velocity i/ is in this prism constant, and belongs to the 

 ray which is polarized in a plane perpendicular to the axis B. 

 The velocity of the other ray depends on the value of s" 

 between the limits \ a. and 90°. Hence the square of the 

 velocity may vary 



between A + B and A + B sin^ \ a. 



3. If the edge of the prism is parallel to the axis C, we shall 

 always have e' = e" + a; hence 



i/^ = A + B . sin- i a, and 

 i!"2= A + B . sin^(V' + \u). 

 In this prism the velocity v' is constant, and belongs to the 

 ray whose plane of polarization is perpendicular to the axis C. 

 As the angle >■" may have different values from 90° — -^ a to 

 — \a., the square of the velocity of the other ray will vary 

 between A and A f B. 

 If in the three prisms, cut as now described, we observe the 

 deviation of the ray, whose velocity remains constant indepen- 

 dently of the direction, and if we calculate the index of refrac- 

 tion, we shall have the values of three quantities A, B and z. 

 Calling n' the index in the prism whose edge is parallel to A, 

 n" that in the prism whose edge is parallel to B, and n'^' that 

 in the prism whose edge is parallel to C, we shall have, the 

 velocity of light in air being taken as unity, 



«'^ = A + B, n'"^ = A, n'"^ = A + B . sin- i a, 

 and consequently 



A = n"', B = n"^ — n'"% and sin^ i a = 



n" — n"' 



I come now to an account of my experiments. 

 Arragonitc. — Out of a crystal of this mineral from Bohemia, 

 I cut three prisms : 



1. The prism A having the edges of its refracting angles 

 parallel to the axis A of the pyramidal cryst&l. A, No. 1, and 

 A, No. 2, are two different refracting angles. 



2. The prisms B had their edges pai'allel to the axis B; the 

 two are marked B, No. 1. and B, No, 2. 



3. The pris7)is C had their edges parallel to the axis C ; two 

 of them thus cut are named C, No. 1. and C, No. 2. The light 

 which moves with a constant velocity may be known by its 

 passing through a plate of tourmaline, having its axis parallel 

 to the edge of the prism. 



Thejirism A, No. 1.— Refracting angle 66° 43' 17". Temp. 

 -f 19° cent. In the spectrum, where the deviations were the 



T2 



