242 PROCEEDINGS OF THE AMERICAN ACADEMY. 



would be possible. Accordingly, some two years ago, Mr. E. H. Colpitis 

 aud the writer undertook the iuvestigation. The methods used and the 

 results obtained were briefly as follows : — 



The concave diffraction grating, slit, aud photographic plate were all 

 enclosed in an air-tight cast-iron box. Light was admitted to the box 

 through a fluorite window. The source of light was a powerful electric 

 spark, obtained from a transformer run from the commercial circuit ; a 

 capacity equivalent to four large Leyden jars was placed in parallel with 

 the spark gap ; magnesium was used for terminals. Following Schu- 

 mann's work, the air was exhausted from the box. The spark was 

 placed close to the fluorite window in order that the column of air 

 between tlie source and the plate sliould be as short as possible. Under 

 tliese circumstances a photograph was obtained showing lines apparently 

 down to wave length 924 ^a, a value lower than Schumann's lowest esti- 

 mated wave length. Terminals of aluminum, cadmium, cobalt, nickel, 

 aud copper were tried. All showed lines below 1500 /a; some showed 

 lines as far down as position 950 /x. 



During these experiments, the air pressure in the apparatus did not 

 always have the same value. Some plates were taken with the pressure 

 as high as 1 cm. ; others with it as low as .2 cm. It was observed that 

 this difference in pressure did not have any effect on the strength of the 

 lines. 



Up to this time the first spectrum bad been used ; now it was desired 

 to find these lines in the second spectrum. In spite of the greatest care, 

 however, no trace of any line below 1900 jx could be found in the second 

 spectrum. 



The absence of the lines in the second spectrum, together with their 

 behavior under small differences of air pressure, raised the suspicion that 

 either the lines were not due to light of a very small wave length, or that 

 light of very short wave length was not absorbed by the air to the extent 

 indicated by Schumann. To test this idea the spark was first removed 

 to a metre's distance from the fluorite window. This air column did not 

 reduce the intensity of these new lines. Next, air was admitted to the 

 box, and still the lines were unchanged in intensity, even through a col- 

 umn of air, now five metres long. Evidently the lines were not absorbed 

 by air. 



Lest the special brand of plate employed was particularly sensitive to 

 the extreme ultra-violet, a number of different plates were tried. In each 

 case a good photograph was obtained, showing practically all the lines. 



A piece of window-glass was placed between the slit and spark, and no 



