436 M. G. Quincke on the Refractive Index of the Metals. 



— is the observed displacement of the interference-bands, 



measured in multiples of the distance between the fringes. 

 This is positive when the light has a less velocity in the metal 

 plate, negative when a greater velocity, than in air. 



If the theory be correct, then the same value for n must be 

 obtained from equations (5) and (9). 



The displacement of the interference-bands in the spectrum 

 of the apparatus employed was now observed near Fraunhofer's 

 line F, so that 



X= 0-0005 millim. 



The displacement — amounted to —0*25 of the distance be- 

 at 



tween the fringes at perpendicular incidence. At the place 

 where the rays had passed through the silver, a bit of iodine 

 was placed, in order to make an estimation of the thickness of 

 the silver at this spot according to the method of Fizeau*. 

 There was formed a layer of iodide of silver, which (looked at 

 perpendicularly) appeared yellow by transmitted, rust-brown by 

 reflected light, and was bordered by three dark rings. This 

 corresponds to a thickness of air equal to 0*001652 of a milli- 

 metre, with Newton's coloured rings. Putting e for the thick- 

 ness of the corresponding layer of air in Newton's coloured 

 rings of a similar tint to the iodide of silver, and, further, 



Equivalent of Silver Ag=107'9 



Equivalent of Iodide of Silver . . IAg = 2349 

 Thickness of the Iodide of Silver . d 1 = 5-602 



Thickness of the Silver . . . . c? 2 = 10*55 



The Refractive Index of Iodide of Silver n x = 2'246, 



then the thickness D of the film of silver out of which the 

 iodide of silver was formed becomes 



J) =rf' C T^ e=0-1086 6 ; . . (10) 



IAg d 2 n x v ' 



or, for this special case, 



D = 0-1086 . 0001652 millim. = 0-0001 788 millim. 



Putting in equation (9) this value for X, D, and — , then 



a 



»= l+ S' ( - M5 )= om 



The correspondence of this value with that found by the other 

 method is much closer than the accuracy of the experiments 

 would lead us to expect. 



* Comptes Rendus, vol. lii. 1. 1861, p. 274. 



