286 



Lord Rayleigh. On the Intensity of [Nov. 18, 



Prism of Grown Glass (I). 



In ordering a glass for the purpose of determining the reflecting 

 power of a surface, a prism was preferred to a plate, both on account 

 of the easier separation of the reflections from the front and back 

 surface, and also because the refractive index could be determined 

 more readily. During the observations the hind surface was coated 

 with black varnish, the effect of which, however, in annulling the 

 second reflection, was far from complete. 



With this glass, carefully cleaned (but not repolished), six sets of 

 observations were made, four by myself and two by Mrs. Sidgwick. 

 Each set consisted of three or four settings with the glass in operation, 

 and about the same number with substitution of the revolving disk. 

 The following is a set of readings by myself on August 7th, 1886 : — 



Face of Prism (I). 

 Beflection. Eevolving disk. 



39 ? 40°3 



38- 2 427 



37-8 43-0 



39- 1 42-0 



— 435 



Mean.. 38-5 42'3 



The angles here given are the obliquities of the adjustable glass 

 plate used to graduate the intensity. According to Pickering's table, 

 calculated from Fresnel's formula (/* = 1'55), the effect of this 

 plate at 38*5° would be to reflect 15*7 per cent, of the light incident 

 upon it. The light transmitted is therefore 84*3 per cent. In like 

 manner the light transmitted by the plate at an obliquity of 42'3° is 

 82*5 per cent. 



In order to complete the calculation of the reflecting power of the 

 glass surface, we must know the proportion of light transmitted by 

 the revolving disk. Measurements gave for the chord of aperture of 

 this disk in fiftieths of an inch 45'0, corresponding to a radius of 

 174-25. The angle of aperture is thus 14° 50' = 14-83°. Accord- 

 ingly the factor expressing the reduction of light by use of the disk 

 is — 



14-83/360 = 0-04119. 

 The reflection from the face of the glass prism is thus — 



0-04119 = 0-0403. 



84-3 



