RECENT EXPERIMENTS WITH INVISIBLE LIGHT.! 
[With 6 plates.] 
By R. W. Woop, LL. D., 
Professor of Experimental Physics, Johns Hopkins University. 
By far the greater proportion of the discoveries which have been 
made in natural science up to the present time depend upon observa- 
tions made with the eye, either with or without the aid of optical 
instruments. The eye is, however, sensitive to only a very small 
part of the total radiation which reaches it, and it seems not unlikely 
that, if its range could be extended, many new phenomena would 
immediately come to light. By the employment of photography and 
of instruments which detect and measure the intensity of the infra- 
red or heat rays, much new information has been gathered, especially 
in the science of spectroscopy; but usually these methods have been 
applied only in cases where the invisible radiations were known to be 
present. It seemed quite probable that if photographic methods 
were applied to various physical phenomena which excluded the 
action of any but invisible rays, new facts would probably be dis- 
covered. I can illustrate what I mean by taking two striking cases 
which were found at the very outset of the investigation, and which 
will be more fully discussed presently. 
If the finger be dipped into powdered zinc oxide and rubbed 
over a sheet of white paper, eye observation is absolutely unable 
to detect the presence of the streaks made by the white powder, 
unless it has been very thickly applied. If, however, we photo- 
graph the paper with ultra-violet light we obtain a picture in which 
the streaks are as black as if made with powdered charcoal. This 
suggests that if we apply the process to the photography of themoon 
and planets, we have some reason to suspect that substances which 
can not be detected visually may come out in the photographs, a 
surmise which has been justified in one case at least. This and 
other similar cases will be taken up in detail presently. 
As an illustration of how the method may be applied to the inves- 
tigation of various physical phenomena, we may take another 
interesting case, in which a new radiant emission from the electric 
spark has been discovered. It was suspected that the very short 
1 Lecture before the Royal Institution of Great Britain, Friday, May 19,1911. Reprinted by permission 
from author’s separate of Proceedings of the Royal Institution. 
155 
