164 



SCIENCE 



[Vol. LV, No. 1416 



bers, but I must confess that I am not alto- 

 getlier satisfied with its validity. As to the 

 members of higlier order adequate proof of 

 their existence seems to be quite lacking. The 

 crux of the whole matter lies in the fact that to 

 produce radiations of sufficiently short wave 

 length a violent disruptive discharge must be 

 employed which introduces impurities torn 

 from the discharge tube and its electrodes. No 

 matter how carefully the gas is treated the 

 lines due to these impurities furnish a con- 

 stant source of confusion which must never be 

 overlooked. In spite of the fascinating possi- 

 bilities conjured up by the work of Rutherford 

 on atomic disintegration, I am of the opinion 

 that but three lines can be ascribed to the 

 spectrum of helium in the Schumann region 

 with any certainty. Of these two lie near 1640 

 and 1215 A.U. and are by no means above sus- 

 picion. The origin of the third at 585 dis- 

 covered by Fricke"^^ and the speaker last year 

 is much more to be relied upon. This last 

 radiation possesses the added importance of 

 showing a direct numerical relation with the 

 radiation potential of helium. 



I have said that interest in our subject is 

 spreading. As evidence I present some very 

 recent results obtained on the Pacific Coast by 

 J. T. Hopfield. He has devised a method of 

 studjdng the emission spectra of gases which 

 are opaque in the Schumann region without 

 the use of a window, the transparent gas which 

 fills the body of the spectroscope and the sub- 

 stance under examination being kept separate 

 by gas currents suitably directed. One of his 

 most striking results relates to the spectrum 

 of oxygen; he not only finds a number of lines 

 throughout the Schumann region which he 

 attributes to this element, but he has also dis- 

 covered that radiations between wavelengths 

 1336 and 990 may be photographed even 

 through a column of oxygen a meter long and 

 at a pressure of about 0.4 mm. This result 

 confirms and extends my observations on the 

 absorption band of this gas. Finally, he 

 appears to have discovered an improved 

 process for the manufacture of Schumann 

 plates. 



" Phil. Mag., 41, May, 1921. 



By far the most important contribution to 

 the subject has been made by Millikan.^- Ably 

 seconded by his students, Sawyer and Bowen, 

 he has not only succeeded in extending the 

 spectrum to the neighborhood of 150 A.U. but 

 also by the study of tbe radiations in this 

 region he has established a connection between 

 light diffracted by a grating and those shorter 

 wavelengths known as X-rays. Rightly con- 

 jecturing that the production of vibrations of 

 the highest frequency depends as much on the 

 intensity of the electric field at the source as 

 upon the substance of the radiator, he has 

 employed a minute high potential spark in 

 the best obtainable vacuum. With this arrange- 

 ment he has investigated the spectra of a 

 number of substances including carbon, zinc, 

 iron, sodium, magnesiiun and aluminium and 

 has measured many of their lines. 



It is a curious fact that many substances 

 produce spectra of striking similarity in the 

 extreme ultra-violet when stimulated by the 

 high potential spark or even when exposed to 

 a disruptive discharge in a vacuum tube. The 

 lines of the spectrum obtained from helium, 

 for example, are nearly all common to the 

 spectra of carbon and of lithium, while 

 aluminium, magnesium and iron, etc., have 

 identical spectra between 1000 and 250 A.U. 

 The presence of a common impurity furnishes 

 the most conservative explanation for this strik- 

 ing phenomenon, though those inclined to spec- 

 ulation may turn a longing eye toward atomic 

 disintegration. Millikan^' has shown that oxy- 

 gen is probably the impurity in question. 



Fabry^* has recently emphasized the fact 

 that this region of roughly 150 A.U. between 

 the limit as set by Millikan's experiments and 

 X-rays capable of analysis by a crystal is one 

 of the most interesting in the whole spectrum. 

 He points out that here the absorption of 

 many substances should pass through a maxi- 

 mum and then decrease; here metals begin to 

 manifest their high refiective power and here 

 the phenomena of refraction will appear, 



^^ Astrophy. Jour., 52, 47, 1920; 52, 286, 1920; 

 53, 150, 1921. 



isProo. Nat. Acad., 7, p. 289, 1921. 



n Journal Franklia Institute, p. 227, 1921. 



