1884.] . Some Experiments on Metallic Reflection. '67 



I had hoped to have made some observations of the amount of 

 light reflected by films of different thicknesses, which I had proposed 

 to obtain in the same way as those used in the experiments of which 

 an account was given in the " Proc. Roy. Soc.," vol. 31, p. 486, but 

 after numerous trials, extending over a period of nearly two months, 

 failed to obtain any suitable films. 



Although the same process was used (Martins, " Ann. de Chim.," 

 4th series, vol. xv, p. 94) the silvered surfaces were never perfectly 

 bright when removed from the solution, as had been the case with 

 those prepared for the experiments already referred to. A few 

 minutes' rubbing with a chamois leather was sufficient to render the 

 mirrors perfectly bright, but the thin deposits of silver obtained by 

 removing the glass plates from the solution before the action is at an 

 end, are not sufficiently coherent to withstand the necessary amount 

 of friction. 



The first set of photometric determinations were made with a 

 silver film deposited on a flat and well-polished glass plate 76'5 

 millims. long and 51 millims. wide ; after being rubbed with a piece 

 of chamois leather the surface was perfectly bright, and when com- 

 pared with the speculum metal mirror in the way suggested by 

 Professor Stokes ("Proc. Roy. Soc.," vol. 35, p. 36) the silvered 

 surface appeared slightly the best of the two. 



The glass plate was weighed before and after being coated with 

 silver, and the weight of the film was found to be 0'0035 grm. ; assuming 

 the density of the silver to be 1O62, that being the value for silver 

 finely divided by precipitation given in " Watts's Dictionary," vol. v, 

 p. 277, the thickness of the film calculated from the area and weight 

 was O00008447 millim., or about the same as the thickest of the 

 films used in the experiments already referred to. 



The film appeared opaque by ordinary daylight, but when examined 

 with sunlight was seen to be slightly transparent and of a deep blue 

 colour. 



The photometrical determinations were made in exactly the same 

 way as those with the speculum metal and steel mirrors (" Proc. Roy. 

 Soc.," vol. 36, p. 187), and the observations were about as concordant 

 as those contained in the Tables I and II of the paper giving an 

 account of the experiments. 



Two complete series of observations were made with light 

 polarised in, and perpendicularly to, the plane of incidence, and the 

 results are given in Tables I and II. 



The angles of incidence are given in the first column, the per- 

 centage amount of light reflected in the second and third, the means 

 of the two sets of observations in the fourth, and the amount of light 

 which ought to have been reflected according to Cauchy's formulas in 

 the fifth. 



