332 On the Affections of Light 



of (m— 1)' in ice to [m'—iy in topaz, m and in' being the 

 indices of refraction. If we take ?«= 1-307 and w/= 1-636, 

 this ratio will be nearly as 8-9 to 1 . In these experiments the 

 two ova! central sj)ots were distinctly seen. 



Light transmitted at an angle of 46° through a plate of ice 

 1-25 inches thick gives rings of the same size as when it is 

 transmitted at an angle of 60^ 38' through a plate of topaz 

 tWo of ^" '"^li thick. By calculating the real thicknesses in 

 the direction of the transmitted light, it will be found that the 

 thicknesses at which ice and topaz produce rings of the same 

 magnitude are as 8-4 to 1, a ratio not very remote from that of 

 {m—\y in ice to (w'— 1)' in topaz. 



Light transmitted at an incidence of 36=" through a plate of 

 ice -p'\f„ of an inch thick gave rings twice as large as those 

 shown by a plate of topaz V^Vo "^ *" '" thick. These thick- 

 nesses will be found, after reduction, to be as 8-2 to 1, a ratio 

 more remote than any of the former from that of (m—iy to 



{m'-xy. 



A plate of ice taken from the surface of a pool of water did 

 not appear to depolarize light, when it was incident perpendi- 

 cularly: but when the angle of incidence was considerable, the 

 light was depolarized in every direction, and the coloured rings 

 appeared even at great obliquities. 



9. Sulphate of potash. A plate of sulphate of potash -^^-^-^ of 

 an inch thick gave fringes of colour, each of which was 4° in 

 breadth, while another plate j'/^«^ of an inch thick gave fringes 

 3° 12' in breadth. Now 



135 :lCiS = 3° 12': 4° nearly, 

 so that the diameters of the rings are inversely as the thick- 

 nesses of the plate*!, as in the case of topaz. The light was in- 

 cident on the sulphate of potash at an angle of 41°, which gives 

 VoW for the oblique thickness of the jilate tttoV of a" inch 

 thick. Now 



_T_'L!'_ . lo tK « 9 . 90 9' 



10 0'' cF 1 -^ Tir"e 15" ' ■'' " i 



the size of the ring that would have been produced by a plate 

 of topaz T-'oVV of an inch thick, so that the thicknesses of sul- 

 phate of potash and topaz that produce equal rings are as 1-85 

 to 1, which is not very far from the ratio oi (m—iy to {in' —\y. 

 If we take 7?z= 1-509 and ?«'= 1-636, this ratio will be as 1'95 

 to 1. 



10. Tartrate of potash and soda. The neutral axes of this 

 salt are parallel and perpendicular to the axis of the prism, and 

 it possesses an oblique depolarizing axis along which the co- 

 loured rings are visible. The thicknesses of this substance and 

 of topaz, at whioh equal rings are produced, are as 31 : 16, 



which 



