168 



MINERALOGY 



angles to each other. Each of these lines represents a direction 

 parallel to which there is a maximum or minimum index of refrac- 

 tion or velocity, for transmitted light. The relative length of these 

 axes will also represent the relative speed of transmission, remem- 

 bering that the velocity is the reciprocal of the index of refraction. 

 Double refraction. A ray of light upon entering an anisotropic 

 crystal or substance, in general, travels with two different velocities 

 within the crystal, or it is broken into two rays, each of which pos- 

 sesses its own index of refraction. In other words, one is a slow ray, 

 the other is a fast ray. The difference between the values of the 

 indices of refraction of the two rays is a measure of the birefrin- 

 gency of the substance in that direction, for the birefringency or 

 double refraction varies with the direction of transmission. 



For calcite one index of refraction = 1.658 and the other = 

 1.486 ; as these are the maximum values, or represent the maximum 

 difference between the two indices, their difference, or .172, would 

 be the double refraction of calcite ; which is very high or strong. 

 In most minerals it is represented by a small figure in the second 

 decimal place, or even in the third, as that for quartz is .009 and 

 that for orthoclase is .007. Since calcite is an example of birefrin- 

 gency in an exaggerated degree, and it is transparent and easily 

 obtained, it is an extremely good mineral with which to demon- 

 strate this peculiar property of crystalline substances. 



The usual cleavage piece of calcite is a rhomb in shape. If such 

 a cleavage piece of calcite be placed over a pinhole in an opaque 

 paper and then held up to the light, two pinholes will appear, 



Fig. 312 ; one will be seen 

 above and nearer than the 

 other; this is due to the 

 difference of the velocities 

 * > e of the two rays. The dis- 

 . > tance between the two 



images will depend upon 

 the thickness of the cal- 

 cite. When the rhomb is 

 revolved, one image e, Fig. 

 313, will appear to revolve 

 around the other, or that ray is refracted to a greater extent 

 than is the other ray. In fact, when the ray of light enters the 

 calcite at right angles to the surface and the eye is in the direc- 

 tion of this ray, when the rhomb is revolved one image is sta- 



FIG. 312. 



