﻿Selective Reflexion ofX 2536 by Mercury Vapour. 1105 



suggestions, which have proved of great service in the 

 revision of the manuscript. The interest he took in this 

 question of the freely equilibrated nucleus and other 

 dynamical problems presented in the paper, provided a 

 valuable stimulus in the course of work which of necessity 

 comprised much tedious calculation. For this assistance 

 I am grateful, and feel all the more indebted since it 

 involved some encroachment on a very scanty leisure. 



CI. Selective Reflexion of \ 2536 by Mercury 

 Vapour. By R. W Wood *. 



IN earlier papers it was shown that there appear to be two 

 types of selective reflexion of radiation which is very 

 nearly in synchronism with the free period of the mercury 

 molecule at the 2536 absorption line. One type is due to 

 the abnormally low value of the refractive index of the vapour 

 on the short wave-length side of the line. The change of 

 refractive index at the boundary quartz-Hg vapour is 

 greater than the change for a boundary quartz-vacuum, 

 since the refractive index is less than unity, consequently 

 we have strong reflexion for radiations immediately adjacent 

 to the absorption line on the short wave-length side. The 

 high value of the refractive index of the vapour for 

 radiations of wave-length slightly greater than that of the 

 absorption line, makes the change of index at the boundary 

 small, consequently the reflexion for these radiations is 

 very feeble. This was shown by reflecting the light of 

 \=2536 from a quartz arc operated at high temperature 

 (2536 broad and strongly reversed) from the inner surface 

 of a flat prismatic quartz plate which was sealed to a quartz 

 bull) containing mercury vapour at a pressure of several 

 atmospheres. The reflected light was photographed with a 

 quartz spectrograph, and only the short w T ave-length half of 

 the 2536 reversed line was found on the plate. 



The experiment was also tried using the 2536 line from 

 the water-cooled quartz arc. In this case the reflecting 

 power of the quartz-mercury vapour surface was about four 

 times as great ;is the normal reflecting power of quartz in 

 this region of the spectrum. Since in this case the light is 

 highly homogeneous, it was inferred that the high reflectivity 

 was the result of the absorbing power of the vapour, the case 

 being analogous to the selective reflexion of the aniline dyes. 



It occurred to me recently, however, that it would be 

 desirable to repeat the experiment using still more 

 homogeneous light, as a considerable portion of the 2536 

 * Communicated by the Author. 



