196 



BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



we shall find that when the axes of both plates coincide, or 

 are parallel to each other, the light which has passed through 

 the first will also pass freely through the second, and they 

 will together appear perfectly transparent ; but when one is 

 turned round, so that the axes of each plate are at right 

 angles, or across each other, not a ray of light will pas/ 

 through ; they will appear perfectly opaque, although we 

 may be looking at the meridian sun. On viewing pieces of 

 selenite or mica through one of these plates in polarized 

 light, they will be seen having the most beautiful colors, 

 which will change by the turning of the plate. 



Price, $2.50, $4.00, and $6.00. 



Fig. 712. 



Tourmaline Polariscope. (Fig. 712.) Two 

 plates of tourmaline are mounted in cells capable 

 of being rotated on their axes, and supported be- 

 fore each other by a spring frame, between which 

 crystals may be conveniently supported, and the 

 system of beautiful rings observed, represented in 

 Fig. 713, 714, 715, and 716. 



Price, $4.00 and $5.00. 



Fig. 713. 



