l;U THE MICROSCOPE. [June, 



Polarized Liglit. 



[Report of a lecture by Prof. John Cox before the Montreal Microscop- 

 ical Society, March 4, 1S92.] 



The lecturer introduced his subject by explaining that common 

 light moves in two planes at right angles to each other, while 

 polarized light moves in one plane only, and that common light 

 might be turned into polarized light, either by transmission or re- 

 flection. It was then shown that a beam of common light could 

 be separated into two beams of polarized light when passed 

 through a crystal having a double refracting power, and as Ice- 

 land spar was the medium generally employed by microscopists, 

 a cr3'stal of that substance was used in demonstrating this fact. 

 Conducting the experiment still further, it was shown how by the 

 addition of another crystal the two beams could be subdivided, 

 and thus four distinct images or disks could be made visible on 

 the screen. By interposing a thin plate of selenite, and rotating 

 the analvzing prism, all those wonderful changes of color were 

 j)roduced, which were so familiar to those accustomed to use the 

 polariscope. The lecturer alluded to the wonderful power which 

 electricitv was known to exert over a beam of light, and illus- 

 trated this portion of his lecture b}' means of a powerful magnet, to 

 which was attached the wires from a battery. The beam of light 

 vvas made to pass through some four inches of glass, affixed to 

 the magnet, and as the current was turned off and on, the effect 

 produced was very apparent. The application of polarized light 

 in the study of crystals was next taken up, and illustrated by plac- 

 inof in the lantern a slide of a certain solution. This was thrown 

 upon the screen, where the crystals could be observed as they 

 formed. vShooting out from all sides and being tinged with in- 

 numerable colors, they looked not unlike a grand display of the 

 aurora borealis in miniature, only with the colors greatly exag- 

 gerated. 



The construction of the Nicol prism and its use vvas then re- 

 ferred to, and the phenomena of Newton's rings explained. 

 Other phenomena connected with light were beautifully demon- 

 strated and explained by means of a mechanical contrivance, con- 

 sisting of a series of stands, having in them movable frames which 

 could be revolved in their sockets. Across these frames were 

 fixed two parallel bars of glass about one-eighth of an inch apart. 

 These represented the different media through which the beam of 

 light was supposed to pass. The beam of light was represented 

 by a silken thread, which was passed through the spaces formed 

 by the glass bars. One end was attached to a small electrical 

 machine, while the other was held tightly at the opposite end. 

 When the current was turned on, the end of the thread attached 

 to the motor was roated in a circuit whose diameter would be 



