314 Lord Rayleigh on the Intensity of Light reflected from 



The surface of water must be large enough to avoid curva- 

 ture due to capillarity. Shortly before an experiment it is 

 cleansed with the aid of a hoop of thin sheet-brass about 

 2 inches wide. The hoop is deposited upon the water so 

 doubled up that it includes but an insensible area, and is then 

 opened out into a circle. In this way not only is the greasy 

 film usually present upon the surface greatly attenuated, but 

 also dust is swept away. The avoidance of dust, especially 

 of a fibrous character, is important. Otherwise the resulting 

 deformation of the surface causes the field of the reflected 

 light to become patchy and irregular. 



We come now to the silvered glass reflectors, which are 

 assumed to reflect the direct and reflected lights equally well. 

 It seems safe to suppose that no appreciable error can enter 

 depending upon the slightly differing angles at which the 

 reflexion takes place in the two cases. But the mirrors are 

 liable to tarnish, and, indeed, in the earlier experiments soon 

 showed signs of being affected. The influence of this tarnish 

 would be much greater in photographs done upon ordinary 

 plates, sensitive principally to blue light, than in the estimation 

 of the eye ; and it was thought desirable to eliminate once for 

 all any question of the effect of differential tarnishing by 

 interchanging the mirrors in the middle of each exposure. 

 For this purpose a somewhat elaborate mounting had to be 

 contrived. It was executed by Mr. Gordon and answered its 

 purpose extremely well. 



The mirrors are carried by a brass tube B (fig. 2), which 

 revolves in an external tube A A rigidly attached to the 

 stand of the apparatus. A lateral arm 0, some inches in 

 length, projects from B, and near its extremity bears against 

 one or other of two screw-stops D. The lower end of B 

 carries perpendicular to itself a brass plate EE (fig. 3). The 

 mirrors GrGr are of plate-glass and are fixed by cement to two 

 brass plates FF. The latter plates are attached by friction 

 only to EE, being on the one hand pushed away by adjusting- 

 screws HH, and on the other held up by four steel springs I. 

 The edges of the reflecting surfaces meet accurately in a line 

 passing through the axis of rotation, and the stops D are so 

 adjusted that the transition from the one bearing to the other 

 corresponds to a rotation through precisely 180°, so that on 

 reversal the common edge of the reflectors recovers its 

 position. The two mirrors were originally silvered in one 

 piece, and the common edge corresponds to the division made 

 by a diamond-cut at the back. These arrangements were so 

 successful that in spite of the reversal between the two parts 

 of the exposure the division line appears sharp in the photo- 

 graphs and exhibits no appearance of duplicity. 



