﻿436 Lord Rayleigh on Departures from 



polishing) in the air did not appear to alter the behaviour 

 materially. On the same plate other bands were treated 

 with hydrofluoric acid — -commercial acid diluted to one-third. 

 This seemed more effective than the putty-powder. At about 

 15° off grazing the spectrum of the reflected light still showed 

 some weakening in the ray of equal index. 



In the cell with parallel faces it is not possible to reduce 

 the angle of incidence (reckoned from the normal) suffi- 

 ciently, a circumstance which led me to revert to the 

 1)0° bottle-prism. A strip of glass half an inch wide could 

 be inserted through the neck, and this width suffices for the 

 observation of the reflected light. But I experienced some 

 trouble in finding the light until I had made a calculation of 

 the angles concerned. Supposing the plane of the reflecting 

 surface to be parallel to the base of the prism, let us call the 

 angle of incidence upon it p^, and let 9, $ be the angles 

 which the ray makes with the normal to the faces, externally 

 and internally, measured in each case towards the refracting 

 angle of the prism. Then 



x = 00°-<£, 4 = sin" 1 (f sin0). 



The smallest ^ occurs when = 90°, in which case ^=18° 10'. 

 This value cannot be actually attained, since the emergence 

 would be grazing. If ^ = 90°, giving grazing reflexion, 

 6>=-48 36 7 . Again, if (9 = 0, %=60°; and if % = 45°, 

 = 22° 51'. We can thus deal with all kinds of reflexion 

 from ^ = 90° down to nearly 18°, aud this suffices for the 

 purpose. 



The strip employed was of plate glass and was ground 

 upon the back surface. The front reflecting face was treated 

 for about 30 s with hydrofluoric acid. It was now easy to 

 trace the effects all the way from grazing incidence down 

 to an incidence of 45° or less. The ray of equal index was 

 in the yellow-green, as was apparent at once from the 

 spectrum of the reflected light near grazing. There was a 

 very dark band in this region, and total reflexion reaching 

 nearly down to it from the blue end. The light was from a 

 paraffin flame, at a distance of about two feet, seen edge- 

 ways. As grazing incidence is departed from, the flame 

 continues at first to show a purple colour, and the spectrum 

 shows a weakened, but not totally absent, green. As the 

 angle of incidence % still further decreases, the reflected 

 light weakens both in intensity and colour. When ^ = 45°, 

 or thereabouts, the light was weak and the colour imper- 

 ceptible. After two further treatments with hydrofluoric 

 acid and immediate examination, the light seemed further 



