Sir John Conroy. 249 



whether it was air or a liquid such as carbon bisulphide or tetra- 

 chloride. The reflection spectra were also studied, and the positions 

 of the absorption bands for varying angles of incidence located, and 

 shown to coincide with the bright spaces in the absorption spectrum 

 of a dilute solution of potassium permanganate. 



The " Distribution of Heat in the Visible Spectrum " ("Phil. Mag.," 

 1879, p. 203) formed the next subject of Conroy's research. From 

 calculations derived from the measurements of Fizeau and Foucault, 

 Lamansky, Knoblauch and Tyndall, it is concluded that the distribu- 

 tion of heat in the normal spectrum differs greatly from that in the 

 dispersion spectrum, and that in the latter the great calorific intensity 

 of the red rays is due to the action of the prism in concentrating these 

 rays upon the face of the thermopile. It was demonstrated that the 

 intensity of the heat in the different portions of the normal spectrum 

 varies but little through a considerable distance, affording some support 

 to Draper's hypothesis that every colour ought to have the same 

 heating effect. 



There next Allowed five papers concerning "Some Experiments 

 on Metallic Eeflection " (" Proc. Roy. Soc.," 1879, p. 242 ; 1881, p. 486 ; 

 1883, p. 26 ; 1884, pp. 36 and 187). In the first, determinations of the 

 angles of principal incidence and principal azimuth, of red light 

 reflected from polished surfaces of gold, in contact with different media, 

 air, water, and carbon bisulphide, are described. The results confirmed 

 Sir David Brewster's opinion that the value of the angle of principal 

 incidence may be taken as indicating the refractive power of a metal. 



In the second paper, a continuation of the experiments for light 

 of other refrangibilities, and for silver plates as well as gold, is shown 

 to have led to the conclusion that Brewster's law that the tangent of 

 the' angle of polarization is equal to the refractive index of the medium 

 holds good for glass in contact with water and carbon tetrachloride, as 

 well as when it is in contact with air ; and in all probability for trans- 

 parent substances in general. Other conclusions were that in the case 

 of metallic films the principal incidence and the principal azimuth 

 increase with the thickness of the film, showing that more than one 

 layer of molecules is concerned in the act of reflection. Also that light 

 polarized perpendicularly to the plane of incidence penetrates to a 

 greater depth than that polarized in the plane of incidence. 



In the third and fourth papers the " Amount of Light reflected from 

 Metallic Surfaces " is discussed. The method employed was to com- 

 pare photometrically the amount of light reflected from a polished 

 metallic surface, inclined at different angles, with that which fell 

 directly on the photometer when the reflecting surface was removed. 

 The experiments showed that the formulae generally received for 

 metallic reflection are approximately correct, but that the actual 



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