

By means of this arrangement the rays reflected from the 

 Inner surface could be studied uncontaminated by the reflexion 

 from the outer surface. 



In the earlier work, the curved surface of the bulb was 

 used, which made experiments on the polarization of the 

 reflected light impossible. 



Z2 



Regular Reflexion of Light by an Absorbing Gas. 331 



rapid circulation of water in the brass jacket keeps the tube 

 quite cold. Asa resonance lamp we have used a quart/, tube, 

 closed by worked plates of fused quartz, containing a drop of 

 mercury, and highly exhausted. This tube was mounted in 

 front of, and close to, the crystalline quartz plate which formed 

 the window of the water-jacket of the lamp. It is important 

 to have the rays of the arc traverse the mercury vapour as 

 near to the front window of the resonance lamp as possible, 

 since it has been shown in one of the previous papers that 

 the intensity of the resonance radiation is reduced to one-half 

 of its value by traversing a layer of mercury vapour, at room 

 temperature, only 5 millimetres in thickness. A screen of 

 black paper, perforated with a hole, cuts off stray radiation 

 scattered by the walls of the resonance lamp, and it is advan- 

 tageous to cover the further end of the tube with a small cap 

 of black paper, or provide some other suitable black back- 

 ground. 



If the invisible light from the resonance lamp is focussed 

 upon a sheet of uranium glass by means of a large quartz lens, 

 we obtain a bright spot of yellow fluorescent light, and can 

 render visible the vapour rising from a warm drop of mercury 

 by holding it close to the screen in the path of the rays, the 

 shadow of the vapour cast on the uranium glass appearing 

 like a column of black smoke, as shown in one of the photo- 

 graphs published in an earlier paper. 



In the present investigation we have used, for the study of 

 the reflexion of the light by the mercury vapour, an exhausted 

 thick-walled bulb of quartz closed by a plate of slightly pris- 

 matic form, as shown by fig. 1. 



Fig-, l. 



