502 



NOTES. 



two optic axes inclined to one another at an angle of 5. A slice of this sub- 

 stance about the 6th or 8th of an inch thick, cut perpendicularly to the axis 

 of the prism, and placed very near to s, fig. 64, so that the polarised ray rs may 

 pass through it, exhibits the system of rings represented in fig. 67, where the 

 points C and C mark the position of the optic axes. When the plate, B, fig. 64, 

 is turned round, the image changes successively to those given in figs. 68, 69, 

 and 70. The colours of the rings are the same with those of thin plates, but 

 they vary with the thickness of the nitre. Their breadth enlarges or diminishes 

 also with the colour, when homogeneous light is used. 



Fig. 69. 



Fig. 70. 



Fig. 71. 



NOTE 209, p. 213. Fig. 71 represents the appear- 

 ance produced by placing a slice of rock crystal in 

 the polarised ray r a, fig. 64. The uniform colour 

 in the interior of the image depends upon the thick- 

 ness of the slice ; but whatever that colour may be, 

 it will alternately attain a maximum brightness and 

 vanish with the revolution of the glass B. It may 

 be observed, that the two kinds of quartz, or rock 

 crystal, mentioned in the text, are combined in the 

 amethyst, which consists of alternate layers of right- 

 handed and left-handed quartz, whose planes are 

 parallel to the axis of the crystal. 



NOTE 210, p. 216. Suppose the major axis A P of an ellipse, fig. 18, to be 

 invariable, but the excentricity C S continually to diminish, the ellipse would 

 bulge more and more; and when C S vanished, it would become a circle, whose 

 diameter is A P. Again, if the- excentricity were continually to increase, the 

 ellipse would be more and more flattened till C S was equal to C P, when it 

 would become a straight line A P. The circle and straight line are therefore 

 the limits of the ellipse. 



