1881.] 



Some Experiments on Metallic Reflexion. 



499 



The table for the silver film in ait* shows that with the increase of 

 the thickness of the film the principal incidence increases for all three 

 kinds of light, but the increase is greatest with red and least with bine 

 light. The principal azimnth also increases with the thickness, bnt 

 does so more rapidly, the difference between the valnes for films I and 

 II and between V and VI being very considerable, whilst the values 

 for films II, III, and IV and VII, VIII, and IX lie close together. 



With films II, V, and VI the azimuths increase from red to bine, 

 whilst with the thicker films they decrease ; film I, however, furnishes 

 an exception, for, although very thin, the azimuths decrease from red 

 to blue. 



In this respect the thicker films behave like the silver plate cleaned 

 with putty powder (p. 496), and the actual values of the azimuths in 

 both cases are about the same ; the principal incidences, however, are 

 much lower with the films than with the plate. The difference in the 

 values of the principal incidences may possibly be due to the effect 

 produced on the plate by the pressure necessary to polish it. 



When the films were in water and carbon tetrachloride, the prin- 

 cipal incidences decreased to a slight extent with the increased 

 thickness, but the variation was small in amount. The azimuths in 

 the case of film I in water increased from red to blue, but in carbon 

 tetrachloride, and with the other films in both liquids, the azimuths 

 decreased from red to blue. 



When the films were in contact with the liquids, the principal inci- 

 dences were lower, except with film I, than in air, and the principal 

 azimuths were also lower, instead of being higher, as had been the 

 case with the gold and silver plates. 



Quincke ("Pogg. Ann.," vol. cxxix, p. 177) made a large number of 

 observations with silver films and red light, and found that the values 

 of the principal incidences increased with tbe thickness, and tended to 

 a constant value, and also that as soon as the thickness of the metal 

 exceeded 0' 00002 millim. there was but little further change in the 

 value of the principal incidence. 



In the short paper which appeared in the " Proc. Roy. Soc," 

 vol. 28, p. 242, it is stated that the numbers obtained by multiplying 

 the tangents of the angles of principal incidence for red light and the 

 gold plate in water and carbon bisulphide, by the refractive indices of 

 these media, were somewhat higher than the tangents of the angles of 

 principal incidence for air, This is also the case for the determinations 

 made with the gold plate and yellow and blue light, and for those made 

 with gold leaf, the silver plate, and the silver films with all three kinds 

 of light. Thus for the silver plate cleaned with putty powder the 

 values are as follow : — 



