RECENT EXPERIMENTS WITH INVISIBLE LIGHT. 1 



[With 6 plates.] 



By R. W. Wood, 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 hi 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 the moon 

 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. 



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