184 



FIRST ORDER 



Grays 



Straw yellow 

 Deep red 



MINERALOGY 



SECOND ORDER THIRD ORDER FOURTH ORDER 



Indistinct 



Purple 



Deep blue 



Light green 



Light yellow Faint red 



Bright red 



Light blue 

 Bright green 

 Yellowish green 



These colors may be compared by use of the quartz wedge, 

 Fig. 334 ; those produced by the thin edge are of the first order, 

 starting with gray of the first order. 



Uniaxial crystals. It has been pointed out that light in pass- 

 ing through a uniaxial crystal is divided into two rays polarized 



and vibrating at right angles to 

 each other, one of which travels 

 with the same velocity whatever 

 the direction, while the velocity 

 of the other varies with the direc- 

 tion. 



If an ellipsoid, Fig. 335, be con- 

 structed in which the three axes 

 are drawn proportional to the 

 Red of the second order, three indices of refraction, which 

 are proportional to the reciprocals 

 of the three velocities, this ellip- 

 soid, in the case of uniaxial crystals, 

 will be one of revolution, every 

 plane section of which will be an 

 ellipse ; there is, however, one di- 

 rection, that perpendicular to the 

 optic axis, in which the plane sections are circles, and light is 

 transmitted in the direction of the optic axis without double re- 

 fraction. The radii vectores of the elliptical section will be a 

 measure of the indices of refraction of the two possible rays 

 passing through the crystal in a path at right angles to the 

 section, and their directions will also indicate their planes of 

 polarization. 



Let Fig. 336 be a section containing the axis of rotation of such 

 an ellipsoid, in which ox represents the smaller index of refrac- 

 tion and oz the larger. Light traveling in the direction of zz' is 

 transmitted with a velocity proportional to oz and an index of re- 

 fraction of ox ; similarly a ray in the direction of xx' is transmitted 



Gray. 



Straw yellow. 



Red of the first order. 



Purple. 

 Blue. 

 Green. 

 Yellow. 



Blue. 



Green. 



Yellow. 



Red of the third order. 



FIG. 334. Quartz Wedge. 



