274 



Sir James Dewar 



[Jan. 16, 



by Provostaye and Desains.* Thus, if there are m surfaces, and 

 y = j — — ) reflected at each surface, then for m surfaces 



II - 1 v- 



my 



1 + [m - 1) y 



will be reflected, and therefore 1 — Y 



1 -v 



transmitted. This expression applied to (1) rock-salt, 

 1 + (m — \)y 



(2) silver chloride, and (3) iodine, gives the values shown in 

 Table VI. 



Table VI. 



In the case of both silver chloride and rubber membranes, as 

 shown by Fig. 18, the observed values diminished more rapidly 

 than these tables indicate. Thus three silver chloride plates only 

 transmitted 4!) per cent, observed as against 70*3 per cent, if there 

 were only reflection losses. When all account is taken of the indi- 



* Ann. de Chim., xxx. p. 159, 1850. 



