1886.] Light reflected at nearly Perpendicular Incidence. 277 



overcome so far as regards the accuracy of the results, but at the cost 

 of several tiresome adjustments, impeding the ready trial and inter- 

 change of various reflectors. 



My apparatus differs in several respects from that generally used 

 for photometric purposes. Before describing it in detail, it may be 

 worth while to indicate some of the considerations which led me to 

 design it. 



The photometers in ordinary use may be said to depend upon the 

 principle of diffusion. If the illuminating candle, or lamp, be drawn 

 back from the screen to double the original distance, the brightness 

 of the screen as perceived by the eye is supposed to be quartered. 

 This implies that (within certain limits) ( the brightness of the screen 

 is independent of the apparent magnitude of the source of light (the 

 total radiation being given), or that the light diffused by the screen 

 in a particular direction (towards the eye) is independent of the 

 direction of incidence. Reciprocally, the light incident in a definite 

 direction is supposed to be diffused through a considerable angle with 

 some approach to uniformity. There is no doubt that with proper 

 arrangements this condition may be satisfied with sufficient accuracy 

 for practical purposes. My object in formulating it is to show that 

 the use of a diffusing screen in photometry is necessarily attended by 

 an enormous reduction of light. 



For our present purpose this loss of light is a serious matter. 

 Weakened to by reflection from glass, the light of an ordinary 

 candle or lamp is hardly sufficient to illuminate a diffusing screen 

 properly, unless placed so close that measurement of the distances 

 becomes uncertain. The difficulty might perhaps be got over by the 

 use of incandescent electric lamps, but such were not at my command. 

 When, as in Sir John Conroy's experiments,* the reflecting surfaces 

 under test are metallic, or when (as above suggested) the observation 

 relates to the transmission of light by an oblique plate of transparent 

 material, the illumination given by a lamp may be adequate. 



In my apparatus all the reflections are regular, and there is no 

 further loss of light than the characters of the surfaces entail. An 

 incidental advantage is that the accurate flatness of surfaces demanded 

 by methods in which illumination is inferred from distance, is here 

 unnecessary. The apparatus was first set up during the summer of 

 1885 ; but the glasses then at my disposal were not good enough, and 

 when the parallel glass mirrors, &c, necessary for satisfactory 

 working came into my possession, the season was so far advanced 

 that I decided to postpone operations until the following summer. 



* ' Roy. Soc. Proc.,' vol. 35, 1883, p. 2G. 



