EXPEKIMENTS WITH INVISIBLE LIGHT — WOOD. 165 



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 light 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 be made by the aid 

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

 rounding an electric spark might possibly be rendered fluorescent by 

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

 Schumann, but that the flourescence 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 

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

 aureole surrounding the spark and extending out in all directions to 

 a distance of nearly 2 centimeters. It was now necessary to prove 

 that this was not light scattered by the dust particles in the air. To 

 do this we have only to protograph the spectrum of the aureole. If 

 it is similar to the spectrum of the spark we are safe in attributing 

 it to scattered light. It it differs we know that it must be fluores- 

 cence, or the genesis of waves of different wave length from any pres- 

 ent in the light of the spark. A photograph of the region surround- 

 ing the spark was made with a quartz spectrograph, and it was at 

 once found that the spectrum was wholly different from that of the 

 spark; in fact, it was almost identical with that of the oxy-hydrogen 

 flame. For the further study of the phenomenon, a piece of appa- 

 ratus was devised by which the light of the spark could be more effec- 

 tually shut off. A small hole was bored through a plate of aluminum 

 fastened to the end of a short vertical brass tube. This plate formed 

 one electrode, the spark passing between an aluminum rod lying 

 along the axis of the tube and the underside of the plate at the point 

 perforated by the hole. 



In a perfectly dark room, if the eye was held a little below the 

 plane of the plate, no luminosity could be seen in the air above the 

 hole, if it was reasonably free from dust, yet a photograph taken with 

 a quartz lens showed a bright beam, or squirt, of light issuing from 

 the hole. A photograph of the phenomenon is here shown, and you 

 will notice the strong resemblance which it bears to a comet (pi. 6, g). 



Many weeks have been spent, in an attempt to determine the 

 exact origin of this radiation, and the question has proved to be the 

 most baffling one which I have ever attempted to solve. The work 



