in Labrador Felspar. 329 



BE. The eye at E will therefore see the reflected image of the 

 candle in the direction ECN, and the mass of coloured light re- 

 flected at A, in the direction EBM. By measuring the angles, 

 I found that when FCR was 78^, the angle NEM, or the dis- 

 tance of the coloured image from the common image, was 57* 

 Calling this distance D, and making m the index of refraction 

 for felspar, A the angle of refraction at C corresponding to the 

 angle of incidence I or FCR, and B, the angle of refraction for 

 a ray EB incident at B (which is equal to the angle of incidence 

 ABw, when the ray passes out of the felspar), and x the inclina- 

 tion of the plane of colour, or CDQ, then we shall have 



. sin I r> sin I D 



sin A > sin B = > 



m m 



and x g *. 



which will give 10 52' for the inclination of the plane of colour 

 to the face of cleavage P, Fig. 3. The common section of these 

 two planes nearly bisects the acute angle of the face P. 



The changeable colours of felspar generally vary from the blue 

 to the red of the second order. In the same specimen, the tint 

 frequently shades off at the edges to the blue of the second or- 

 der ; and when we view it at an oblique incidence, by cement- 

 ing a prism on the polished surface, they diminish from the 

 maximum tint to the blue, and sometimes to the purple of the 

 second order. The colours are not produced by a single plane, 



* The demonstration of this is very simple. Through C and B draw Bw, and 

 FCQ perpendicular to DC, and through A draw AF perpendicular to DQ, and 

 meeting En in TO. Then a; CDA = AFQ = AwB, and BAF = ABn + A wB = 

 B + x. But FAC = ACQ AFC ; consequently, since FAC = BAF, and 

 ACQ = A, we have 



BAE = B + x t and BAE = A x. Hence, 



A B 



B + x = A ,-c, and x = 5 



