172 M. G. Quincke on the Optical Properties 



which passed through the metal ; hence the velocity of light in 

 these metals is greater than the velocity in air. 



Gold may be investigated without the use of any support of 

 glass, if a rectangular piece of gold-leaf be suspended vertically 

 in the air. The experiment gives the same result either with or 

 without glass. 



Since, in order to be transparent, the metallic films must not 

 be thicker than about one-fifth the length of an undulation, it 

 follows that the displacement of the interference-bands never 

 exceeds 0*1 or 0*2 of the interval between the interference- 

 bands. 



But, as Faraday has shown, silver and gold occur in very dif- 

 ferent modifications. Transparent silver of a yellow or grey 

 colour causes a displacement which is neutralized by an accele- 

 ration of the ray passing through the metal ; that is, the light 

 passes with less velocity through the metal than through air. 

 The same is the case with many varieties of gold and gold-leaf. 

 Gold and silver metallic plates, through which light passes with a 

 greater velocity than through air, become modified by simple 

 standing in the air, so as to transmit light with less velocity than 

 it is transmitted by air. This should excite the less astonishment, 

 since Vogel* has noticed a spontaneous alteration of silver, and 

 found that the mere contact of acids which occasion no chemical 

 action turns the blue colour of silver immediately into grey or yel- 

 low. Yellow silver behaves towards reflected light also differently 

 from blue silver. Since, moreover, the yellow varieties of silver, 

 and the red or orange varieties of gold, become transformed by 

 pressure into those which are transparent and blue or blue-green 

 in colour, it follows that polished silver or gold must display the 

 properties of the blue silver or blue-green gold, and have an index 

 of refraction n < 1 . This accords with the result at which Beer 

 and Eisenlohr have arrived by applying the theory to the observa- 

 tions of Jamin on the light reflected from polished silver. 



With transparent platinum plates there was always a displace- 

 ment of the interference-bands of the spectrum which could be 

 destroyed by an acceleration of the ray passing through the 

 metal ; i. e., light passes through platinum with less velocity 

 than through air. 



The observations of the author upon the principal incidence A 

 and azimuth H of the light reflected from polished gold or pla-. 

 tinurn plates, gave values from which, with the aid of Eisenlohr' s 

 formula?, there came out n < 1 for gold, and n > 1 for platinum, 

 as is shown by the following Table, in which the former nota- 

 tion is retained : — 



* Pogg. Ann. vol. cxvii. p. 318. f Ibid. vol. civ. p. 374. 



