250 Obituary Notices of. Fellows 



intensity of the reflected light is always less than the theoretical 

 intensity, and that probably three constants are necessary to define a 

 metal optically. In the fifth communication on this subject similar 

 results obtained with films of silver deposited on glass are described. 



Another memoir, arising out of the work on metallic reflection, is 

 entitled "A New Photometer" ("Phil. Mag.," 1883, p. 423). InitConroy 

 describes a modification of Ritchie's photometer, for which twice the 

 accuracy of Bunsen's disk is claimed. Instead of the ordinary form 

 of Ritchie's photometer, consisting of two pieces of white paper 

 fastened to the adjacent sides of a triangular block of wood, each 

 illuminated by one only of the lights to be compared, Conroy places 

 one of the pieces of paper slightly in front of the other and over- 

 lapping it to a certain extent, so that, while both are visible to the 

 observer, each is illuminated by only one of the sources of light. 

 When equally illuminated, the edge of the front paper vanishes. 

 With this photometer the results on metallic reflection were obtained. 



Next followed an important memoir " On the Polarization of 

 Light by Reflection from the Surface of a Crystal of Iceland Spar " 

 ("Proc. Roy. Soc.," 1886, p. 173). Brewster had found, in the year 

 1819, that the angle of complete polarization of the light reflected 

 from Iceland spar depends on the position of the reflecting surface, 

 relatively to the crystal axis, and upon the relation of the principal 

 section to the plane of reflection. Conroy 's experiments with cleavage 

 faces of Iceland spar in air, water and carbon tetrachloride, confirm 

 the accuracy of Brewster's observations for media other than air, and 

 show also that, as pointed out by Seebeck, the change in the value of 

 the azimuth of the plane of polarization of the reflected light occurs 

 to a less extent when the crystal is in air than when it is in denser 

 media ; and that, as the refractive index of the medium increases, the 

 change becomes greatly augmented. It was demonstrated that the 

 facts of the case are accurately represented, for air or water as media, 

 by Brewster's formula for the angle of polarization A' = A + Sin 2 a 

 (A" - A), in which A and A" are the maximum and minimum 

 polarizing angles (that is, in the azimuths of and 90), and A' the 

 polarizing angle at any intermediate azimuth. 



Three years later there followed a memoir on " Some Observations 

 on the Amount of Luminous and Non-luminous Radiation Emitted 

 by a Gas Flame " (" Proc. Roy. Soc.," 1889, p. 55). The most important 

 conclusions contained in this paper are that the radiation from an 

 Argand gas burner consists of about 175 per cent, of luminous, and 

 98*25 per cent, of non-luminous radiation; and that there is no 

 difference, measurable with a thermopile and galvanometer, between 

 the diathermancy of pure water and of a solution of alum. This latter 

 fact is of considerable practical importance, for it shows that a simple 



