408 





Prof. E. W. Wood on the 

 Smoke-Film. 



n. 



X. 



1lK. 



•853 



663 



566 



•923 



605 



558 



•960 



566 



543 



1-037 



520 



539 



1064 



479 



510 



1-124 



436 



490 



1-149 



414 



476 



In the case of the films deposited by the lamp, which do 

 not have the exceedingly porous structure of the smoke-films, 

 Ave can easily determine the thickness by substituting the 

 half-coated plate for the back mirror of the interferometer, 

 and measuring the shift of the fringes at the edge of the 

 reflecting carbon-film. A displacement of one third of a 

 fringe was found with sodium light. The thickness was 

 therefore ^X J X '000589 or -000098 mm. The displacement 

 of the sodium-light fringes corresponding to four transmis- 

 sions (two films in optical juxtaposition being used as described 

 in the paper on cyanine) was 0'784 of a fringe, or 0'196 for 

 one transmission. We have then, substituting in the formula 



Ou,-l)-000098 = -196x -000589 

 ^-1 = 1-2, 



or the refractive index //, = 2*2, not very far from that of the 

 diamond. 



Fig.l, 



I PLANE P/iR.PLATE|CUl?f ■. S=? 



'TZfc/TZ^m- 



r ;.: 



Formation of smoke-prisms. 



One instinctively places more reliance on determinations 

 of refractive index made by prismatic deviation than on those 

 based on interference methods, and an attempt was accord- 

 ingly made to verify the results with carbon prisms. By 

 arranging a very small pointed gas-flame close to the edge 

 of a glass plate> and sliding a piece of plane parallel optical 

 glass back and forth against a guide (the tip of the flame just 



