1883. | Some Experiments on Metallic Reflection. 



37 



black screen, in order to prevent any light diffused from the observer's- 

 face being reflected by the plate into the canister, and thus destroying 

 its perfect blackness. 



Two of the plates were placed side by side and examined together, 

 and in this way it was ascertained that their order of polish was, 

 steel, speculum metal, silver, and tin ; there being but little difference 

 between the polish of the steel and the speculum metal, and a con- 

 siderable difference between the speculum metal and the silver, and 

 again between the silver and the tin. 



The experiment was originally made with the silver mirror polished 

 with putty powder, and was repeated after it had been polished with 

 rouge ; the order remained the same, but it was thought that there 

 was a greater difference between the speculum metal and the silver in 

 the latter case. 



Professor Stokes also suggested that the surface of perfectly clean 

 mercury would furnish a standard. Some mercury was, therefore, 

 cleaned by being well shaken with pounded sugar, and then filtered 

 three or four times through a cone of writing paper, with a small 

 aperture at the apex. A small porcelain basin was blackened both 

 internally and externally, and nearly filled with the clean mercury, 

 and the reflection of the canister in it and in the plates compared. 

 The reflection in the steel appeared quite as black as that in the 

 mercury, whilst that in the speculum metal appeared slightly less 

 black. In a preliminary experiment the steel was thought to be 

 blacker than the mercury ; the difference, if any, was, however, very 

 slight, and was probably due to a thin film having formed on the 

 surface of the mercury which, in that case, had only been cleaned by 

 filtration through paper. The experiment was therefore repeated with 

 mercury which had been cleaned as has already been described, but it was 

 still found that, as compared with the mercury, the polish of the steel 

 was sensibly perfect. The difference between the calculated and 

 observed results for the steel at least cannot, therefore, be due to 

 imperfect polish, and the discrepancy is almost too great to be 

 accounted for by errors of observation, especially as the intensities 

 actually observed increase with the incidence, whilst theoretically they 

 ought to diminish, and then increase again. 



The experiments show (unless there is some error due to the 

 method of observation, and therefore common to all the deter- 

 minations), that the amount of light reflected by silver, steel, and tin 

 gradually increases with the angle of incidence ; that with speculum 

 metal after first increasing it diminishes slightly, and then increases 

 again ; that the results obtained by the method described in this paper 

 are not in accordance with the experiments of Potter and M. Jamin, or 

 with the values calculated by the formulae of Cauchy and MacCullagh ; 



