﻿1106 Selective Reflexion of A 2536 by Mercury Vapour. 



radiation, even in this case, can pass through a layer of 

 mercury vapour at room temperature, 10 cm. in thickness. 

 We may thus interpret the result of the experiment as due- 

 to the selective reflexion of a certain very narrow range 

 of wave-lengths in the 2536 line, just as in the previous 

 case where the high temperature quartz mercury arc was 

 used. I have accordingly repeated the experiment using a 

 mercury resonance lamp at room temperature as a source of 

 light. 



As I have shown in previous papers, the radiation in this 

 case is almost completely stopped by a layer of mercury 

 vapour at room temperature a few millimetres in thickness. 



The thick- walled bulb of fused quartz closed at one end by 

 a prismatic plate of the same substance, which was used in 

 the earlier work, was mounted in an electric furnace in close 

 proximity to a thermo-couple. 



The resonance lamp was mounted in such a position that 

 its image, reflected from the inner surface of the prismatic 

 plate, was received by the lens of the quartz camera. This 

 adjustment was facilitated by attaching a small square of 

 white paper to the surface of the resonance lamp, in 

 coincidence with the area which radiated the 2536 mono- 

 chromatic light, when the lamp was illuminated by the 

 concentrated beam of 2536 light from a quartz monochro- 

 mator, the light coming originally from a water-cooled 

 quartz mercury arc. 



The paper square was illuminated by a concentrated beam 

 of white light, and by carefully adjusting the bulb in the 

 furnace, the image of the paper, reflected from the inner 

 surface of the plate, was seen in the camera. The paper 

 square was then removed, and two exposures made, with the 

 resonance lamp in operation, one with the bulb cold, the 

 other with the bulb at 400°, the photographic plate being 

 moved between the exposures. Just before each exposure 

 the prismatic plate of the bulb was super-heated, by brushing 

 it with a small pointed gas flame. This removed any 

 condensed droplets of mercury, which sometimes formed 

 on the inner surface of the plate. 



Tiie image reflected from the plate was much denser in the 

 case of the exposure with the bulb hot. A number of 

 exposures were now made giving longer times for the cases 

 in which the bulb was cold. These showed that the 

 reflecting power of the plate when backed by dense mercury 

 vapour (density corresponding to 400°) was between 3'5 

 and 4 times as great as the normal reflecting power of 

 quartz for the wave-length in question, a result which is ini 

 agreement with the value found in the earlier work. 



