248 



Transaciions of the 



Fig. 2. 



making the prism is 1 • 52G, we find the value of x, representing 

 the angles of the prism, to be 43:^''. 



The formula being general, is applicable to glass of any density, 

 or to any metal which can be thrown down on the glass, by simply 

 substituting for a and n their resj^ective values, but silver being 

 more brilliant than any other metal, it is probable that, lor micro- 

 scopical purposes, no better can be employed. 



Having thus determined the angles, we have now only to ascer- 

 tain the size of the prism, by which I mean its length relative to its 

 thickness. When the reflecting metal is silver, it depends exclu- 

 sively on the value of n, and when this equals 1 • 526 the ratio of 

 the thickness to the length of the prism will be as 1 to 4-122, and 

 for all practical purposes this is sufficient ; if greater accuracy is 

 required, which is hardly possible, the new measure can readily be 

 ascertained by the ratio of the sines to the sides of the triangles. 



Generally speaking, a prism whose angles are 43|° should be 

 about 1^ inch long and three-eighths of an inch in both width and 

 thickness ; but this must of course depend in a 

 great measure on the size of the Nicol which is 

 used as the polarizer. 



In the instrument before you, the prism — 

 which is 2i^yth long and | an inch thick — is 

 inserted in a brass tube, Fig. 2, on the upper 

 end of which a small condensing lens of short 

 focal length has been screwed for the purpose of 

 condensing the light, and it being desirable at all 

 times to know in what plane the reflecting surface 

 is placed, a small point of german silver is made 

 to project from the tube at right angles thereto ; 

 and still further, as a matter of convenience, for 

 the same purpose, the outside of the tube is black- 

 ened for its whole length and half its diameter. 

 Thus prejjared, nothing is required but to place 

 the tube containing the prism in what is called 

 the selenite fitting of Beck's polarizer, by which 

 it can be placed in any azimuth. 



On the stage of the microscope is a film of 

 selenite of uniform thickness, which has been 

 divided in a line midway between the two neutral 

 axes, the cut portion being brought together after 

 inverting one of them, by which means a com- 

 pound film has been formed, which exhibits simul- 

 taneously, as pointed out by Sir Charles Wheat- 

 stone, both right-handed and left-handed successive polarization — 

 when the polarizer is turned, the tints of one portion ascend, while 

 those of the other descend, and when it is at 45'^ and 135'^, they 



t 



