18H Professor R. W. Wood [May 19, 



photographed dark in ultra-violet light, and found that it contained 

 iron and traces of sulphur. Photographs of rocks stained with iron 

 oxide did not show the required peculiarity, and I accordingly attri- 

 buted the result to the sulphur. A light deposit of sulphur was 

 forraed on the surface of a piece of light-grey rock by directing a 

 fine jet of sulphur vapour against it. Tbe deposit was so slight that 

 absolutely no trace of it could be detected by the eye. The specimen 

 was then photographed with yellow, violet, and ultra-violet light, and 

 it was found that the deposit was quite invisible in the first picture, 

 faintly visible in the second, and quite black in the third — precisely 

 the peculiarity shown by the deposit surrounding the crater Aris- 

 tarchus. Fig. 8, a, b, c, show the gradual appearance of the deposit, 

 which is an oval spot in the centre of the specimen. I feel inclined, 

 therefore, to attribute this spot to an extensive deposit of sulphur, 

 resulting from vapour ejected from the crater. The shape and 

 vast extent of the deposit has always suggested to me that it resulted 

 from material driven out in a volcanic blast.] 



Eeturning now from the moon to the physical laboratory, we will 

 consider a further phenomenon which has been discovered and studied 

 by means of photography in the ultra-violet region. The vapour of 

 mercury has an absorption-baud in this region at wave-length 2o36, 

 which I have made the subject of a somewhat extended investigation. 

 At low pressures the line is very narrow, resembling one of the D 

 lines of sodium, and I have detected its presence in mercury vapour 

 at room temperature, by employing a tube 3 metres long closed with 

 quartz plates. It occurred to me that this vapour might prove to 

 be the substance which I have long sought for the study of what I 

 liave named resonance radiation, i.e. a re-emissicn of light by aljsorb- 

 ing molecules, of precisely the same wave-length as that of the light 

 absorbed. Sodium vapour was found to exhibit the phenomenon, but 

 the experimental difficulties were so great that very little was accom- 

 plished. A small box was made of brass and square plates of quartz. 

 The inside was varnished and blackened with soot, a drop of mercury 

 introduced and the box exhausted. The camera with its quartz 

 objective was now trained on the box, and a beam of light from a 

 mercury lamp (quartz) focused at the centre of the box. Though the 

 eye could see no trace of the cone of rays, the photograph brought it 

 out as distinctly as if the box was full of smoke. An exposure of only 

 one second was necessary, and with a ten-second exposure the spectrum 

 of the light scattered by the vapour was secured. It was found to 

 consist of a single line only (the 2536 line), though the light entering 

 the box was the total radiation of the mercury arc, the spectrum of 

 which contained hundreds of lines. The pressure of the mercury 

 vapour was about 0*001 mm., in other words, ytnrVijTj of the pressure 

 of the air in the room. It seems most extraordinary that a vapour 

 at such a very low pressure and at the temperature of the room should 

 glow so brilliantly with invisible light. A little further experiment- 



