26 



Sir J. Conroy. 



[Feb. 15, 



February 15, 1883. 



THE PRESIDENT in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The following Papers were read: — 



I. " Some Experiments on Metallic Reflection. No. III. On the 

 Amount of Light Reflected by Metallic Surfaces." By 

 Sir John Conroy, Bart., M.A. Communicated by Pro- 

 fessor Stokes, Sec. R.S. Received November 1, 1882. 



As far as I am aware the only experiments that have been made on 

 the amount of radiant energy reflected by metallic surfaces at different 

 angles are those of Potter (" Edinburgh Journal," vol. iii, 278) and 

 Jamin (" Ann. de Chim. et de Phys." [3J, xix, 296) for light, and for 

 radiant heat, those of Forbes (" Phil. Mag.," [3], viii, 246) and of 

 MM. De la Provostaye and Desains ("Ann. de Chim. et de Phys." 

 [3], xxx, 276. 



Potter used a Bouguer's photometer ; the transparent screen being 

 made of white paper, behind which two lamps were placed, the light 

 from one of which always fell directly upon one half of the screen, 

 whilst that from the other either fell directly upon the screen, or after 

 reflection from the metallic plate. The observations were made by 

 measuring the distance at which the first lamp had to be placed in 

 order that both halves of the screen should appear equally bright. 



The two illuminated portions of the screen were not actually in 

 contact, being separated by a dark shadow, an arrangement which, to 

 a certain extent at least, must have interfered with the accuracy of 

 the determinations. The mirror having been placed close to the 

 lamp, the light incident upon its surface must have been very 

 divergent, and as, in addition, the angle at which it was placed could 

 not, owing to the construction of the apparatus, have been very 

 accurately determined , the values of the angles of incidence can only 

 be considered as approximations. 



In M. Jamin's experiments the reflecting surface was half glass 

 and half metal, the line of separation being vertical, and the incident 

 light polarised in a plane 45° to this direction ; the reflected light was 

 examined with a double image prism which was rotated until one of 

 the images due to the light reflected by the metal, and one of those 



