r.»ll] Oil Recent Experiine/i/s irith Invisible Light. ls:» 



[wi resulted in a further discovery. It whs found that if the box 

 was tilled with air at atmospheric pressure, the cone of rays ijluwed 

 feebly in the mercury vapour with which the air was saturated. As 

 the pressure was reduced the glow increased in brilliancy, reaching 

 its maximum at a pressure of about 5 mm. As the exhaustion was 

 pushed further the mercury vapour outside of the cone became 

 luminous, and at the highest vacuum attainable the glow filled the 

 entire box. This is secondary resonance radiation excited by the 

 primary radiation of the mercury vapour, which is excited by the 

 cone of focused rays. The brilliancy of the cone remained about 

 the same, so th.n.t we cannot attiibute the bursting out of this 

 secondary fluorescence to a mere increase in the brilliancy of the 

 directly excited vapour. 



Experiments are now in progress to determine why the presence of 

 a few millimetres of air destroys all trace of the secondary radiation. 

 Photographs of the glowing vapour in air at pressures of 5 mm., 

 1 mm., and are reproduc(!d in Fig. s, d, e,f. 



If we put the drop of mercury in a small flask with very thick 

 walls, exhaust the air, and seal the neck of the flask with the oxy- 

 hydrogen flame, we are in a position to study this interesting type 

 of radiation in mercury vapour at high pressures. I found that as 

 the temperature of the flask was raised the radiation came from a 

 region nearer and nearer tlie front surface which was illuminated 

 by the rays from the lamp, and that when the pressure was about 

 ten atmospheres the ray from the lamp, which had a wave-length of 

 2536, was selectively reflected from the surface of the vapour, pre- 

 cisely as if the inner surface of the bulb were plated with silver. 

 The other rays passed through the bulb with their usual facility. I 

 am at the present time engaged in the study of just how the change 

 from the resonance radiation (which is scattered in all directions) to 

 the regular reflection takes place, a matter of great interest in con- 

 nection with the theory of absorption and reflection. As a matter of 

 fact, I expect it to turn out that the mercury Hght does not absorb the 

 light at all, for experiments indicate that the lateral emission of the 

 ultra-violet light is about as bright as when white paper is used to 

 scatter the light. 



Another interesting line of investigation which I have recently 

 carried out illustrates how new discoveries may l)e made by the aid 

 of ultra-violet photography. It occurred to me that the air sur- 

 rounding an electric spark might possil)ly be rendered fluorescent by 

 the absorption of the very short ultra-violet waves discovered by 

 Schumann, but that the fluorescence might be made up wholly of 

 ultra-violet light, and consequently invisible. I therefore photo- 

 graphed the region surrounding a powerful spark discharge with a 

 quartz lens, shielded from the direct light of the spark by a circular 

 disc. The photograph, when developed, showed a highly luminous 

 aureole surrounding the spark and extending out in all directions to 



