OPTICAL INSTRUMENTS. 

 Fig. 721 



199 



Polariscope with Selenite Designs. (Fig. 724, next page.) 

 This polariscope with selenite designs, consists of a black 

 glass mirror for a polarizer, which is mounted in a frame 

 movable on an axis, with screws to tighten, and having a 

 frame over the mirror to support the slide containing the 

 selenite designs, six of which accompany the instrument. 

 They are viewed through a Nichol's prism set in a cell rotat- 

 ing in a tube to be held by the fingers. The forms of 

 some of these beautiful designs are represented in Figs. 717 

 to 722. The tints are of the most gorgeous description, 

 presenting every variety of color and shade. 



Price, $25.00. 



Double Refraction communicated to Fi - 725 - 



Plates of Glass by Mechanical Force. 

 (Fig. 725.) This apparatus for 

 showing the transient polarizing 

 structure communicated to a piece of 

 annealed glass by pressure, consists of a plate of glass well 

 annealed, from one to two inches square, and about one- 

 third of an inch in thickness, contained in a brass frame be- 

 tween two rounded pieces of steel, having a screw by which 

 any required pressure may be given to the glass ; the polar- 

 izing structure is to be viewed by means of a tourmaline, or 

 Nichol's prism, when the system of fringes appear with 

 thair colors. Price, $5.50. 



