EXPERIMENTS 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- 
eraphed 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 (pl. 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 
