230 SUPPLEMENT TO THE 



manifest ; also by holding it up and causing it to form an 

 image of a candle, so close to the eye that the whole disc 

 of the lens shall appear illuminated, which will be when 

 the image of the candle falls on the cornea; defects of 

 polish and impurities, and seediness, &c. in the cement 

 uniting glasses, will become evident by holding the lens so 

 that the light of a candle shall fall obliquely upon it, and 

 may be farther verified by examining it as a microscopic 

 object. 



A flat piece of flint glass may be examined by looking 

 through it in the direction of its thickness, also by cement- 

 ing a plano-convex lens of plate glass to it, so as to give it 

 the power of refraction, as if the whole was a solid object- 

 glass, and treating it accordingly. 



POLARIZATION OF GEMS AND JEWELLED MICROSCOPES. 



The polarization of precious stones may be seen by 

 working a small portion of them flat, to which a small 

 aperture may be cemented, through which a candle must 

 be viewed in a dark room ; when, if there is no polariza- 

 tion, but one image will be visible 5 but more than one, if 

 it exists. A plano-convex lens may also be cemented to a 

 plate formed from a precious stone, and the effect tried in 

 this way, on a bat's or mouse's hair, which will present 

 several images, if double refraction is present; or the 

 whole may be made to form a miniature image of a candle, 

 as the object-glass of a little telescope, which will give 

 only one image of the taper, if the refraction is single. A 

 plate of gem may moreover be tried as to polarization, by 

 placing it between two tourmalines, which will be sure to 



