242 PROCEEDINGS OF THE AMERICAN ACADEMY. 



would be possible. Accordingly, some two years ago, Mr. E. II. Col pitta 

 and the writer undertook the in \ «--t i - :tt i. >n. The methods used and the 

 results obtained were briefly as follows : — 



The concave ditfraction grating, >lit, and photographic plate were all 

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

 throngb a fluorite window. The ><>urce of light was a powerful electric 

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

 capacity equivalent to lour 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 the source and the plate should be as short as possible. Under 

 these circumstances a photograph was obtained Bhowing lines apparently 

 down to wav.- length 924 /x, a value lower than Schumann's lowest esti- 

 mate'! wave length. Terminals of aluminum, cadmium, cobalt, nickel, 

 and copper were tried. All showed lines helow 1.300 /a; some showed 

 lines as tar down as position 950 /x. 



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

 always have the Bame 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 

 hues. 



Up to this time the first spectrum had 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 /a could he found iu the second 



Bpectrum. 



Tin- absence of the Hues in the second spectrum, together with their 

 behavior under -m ill differences of air pressure, raised the suspicion that 

 either the lines were not due to lightofa very Bmall wave length, or that 

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

 indicated 1>\ 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 -till the lines were unchanged in intensity, even through a col- 

 umn of air, now five met re- 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 plate- were tried. In each 



:t good photograph was obtained, -bowing practically all the lines. 



A piece ofwindow-gla placed between the Blil and Bpark, and no 



