Rays, in Ctystals 'with one and two Axes ofDoiibleBefraction. 141 



to B. That the velocity of the rays in the other spectrum 

 varies, is proved by the two following observations. 



Prism B, Prism B, 



No. 1. No. 2. Diff. 



H . . 1-54242 . . 1-54277 . . O'0O035 

 G . . 1-53493 . . 1-53529 . . 0-00036. 

 Prism C, iVo.l.— Refracting angle 29° 43' 21". Temp. + 17°. 

 The ray H was in both spectra brought to a minimum devia- 

 tion. 



Prism C, No. 2.— Refracting angle 41° 34' 32". Temp. + 16". 

 In the spectrum of greatest deviation the ray F, and in the least 

 the ray H, was brought to a minimum deviation. 



The following were the refractive indices in the spectrum, 

 whose plane of polarization was pei'pendicular to the axis C. 

 Prism C, No. 1. Prism C, No. 2. 



H . . 1-70512 .... 1-70505 

 G . . 1-69830 .... 1-69843 

 F . . 1-69049 .... 1-69058 

 E . . 1-68634 .... 1-68635 

 D . . 1-68157 .... 1-68156 

 C . . 1-67777 ... . 1-67781 

 B . . 1-67632 .... 1-67630 

 The indices vary in the other spectrum, as the following 

 observations show. 



Prism C, No. 1. Prism C, No. 2. 



H . . 1-55043 .... 1-56158 



F . . 1-54265 .... 1-55331. 



All these observations incontestably confirm the fundamental 



theorem of Fresnel, that the velocity of one ray is invariable 



as long as its plane of jjolarization remains the same. 



The following means of the two systems of indices for the 

 three spectra, whose planes of polarization are perpendicular 

 to the three axes of crystallization, exhibit the elements of re- 

 fraction of ana":onite. 



Calling 71 ', n' and n" the indices for the spectra polarized 

 perpcnilicularly to the axes A, B and C, and calculating the 



7t"' w"' 



ratios — ;- and —jrt we shall find 

 «' n" 



