138 



SCIENCE 



[N. S. Vol. L. No. 1284 



Me. G. G. Henderson, M.A., D.Sc, LL.D., 

 has been appointed to be Eegius professor of 

 chemistry in the University of Glasgow, in 

 the room of the late Professor John Ferguson. 



DISCUSSION AND CORRESPONDENCE 



THREE FOURTHS OF AN OCTAVE FARTHER 

 IN THE ULTRA-VIOLET 



In the Physical Review for August, 1918, 

 Vol. 12, p. 167, we made a preliminary report 

 upon a new method of obtaining grating- 

 spectra in vacuo devised by one of us in the 

 expectation of extending the limits of the 

 ultra-violet spectrum. This report was made 

 because both of us were engaged in war 

 activities, and could not then push further 

 the very signiiicant results which we had 

 already obtained — results which brought to 

 light some 30 new zinc lines, the shortest 

 wave-length among which had a value of 928 

 Angstroms. 



There is every reason to believe, however, 

 that every element except hydrogen will emit 

 line spectra corresponding to waves of higher 

 frequency than this, the limiting frequency 

 for a given element pushing farther and 

 farther into the ultra-violet, the higher the 

 atomic weight of the element.^ With a prop- 

 erly chosen source therefore, the limit to the 

 observable ultra-violet spectrum ought to be 

 set solely by the properties of the grating and 

 by those of the medium through which the 

 radiation passes. Heretofore, it has been set 

 by the limitations of the source and the 

 properties of the absorbing medium. We felt 

 that we had removed these limitations en- 

 tirely by working in a very high vacuum with 

 a type of source altogether new in vacuum 

 spectrometry, and one which enabled us to 

 use enormous energies in the highest attain- 

 able vacuum. In our preliminary report we 

 stated that we had " indications of zinc lines 



of shorter wave-length than 928A though no 

 positive proof as yet." 



Immediately upon release from the service 

 we had a new grating constructed so as to ob- 

 tain the maximum possible brilliancy, and a 

 new and very efficient diifusion pump, so as 

 to eliminate altogether, if possible, the ap- 

 pearance of all glow discharges and enable 

 very high potentials (up to several himdred 

 thousand volts) to be used in producing our 

 hot sparks in vacuo. We hoped thus to bring 

 up the intensities of the very short lines. 

 We also eliminated frojn the vacuum chamber 

 certain gas evolving bodies like ebonite which 

 had appeared to limit our exposure times by 

 reducing the periods during which we could 

 operate our hot sparks without giving rise to 

 glow discharges, and which in addition had 

 very injurious effects upon our grating. 



As a result of these improvements we are 

 now maintaining an exhaustion of about 10"* 

 mm. of mercury while the arc is running. 

 We have thus brought to light a considerable 

 number of new zinc lines below 928A of such 

 wave-lengths as to add up to date three 

 fourths of an octave to the ultra-violet spec- 

 trum directly accessible to study with a 

 grating spectrometer. We shall be in posi- 

 tion at a very early date to publish a series of 

 actual photographs, but in this preliminary 

 report will content ourselves with stating that 

 we have ten defuiite reproducible zinc lines 

 below 500 Angstroms the shortest having a 

 wave-length of 320 Angstroms. It is inter- 

 esting to note by reference to the accompany- 

 ing figure which is an extension of one given 

 by Lyman- that this represents an extension 

 in frequency of about four times that accom- 

 plished by Schmnann, namely, 82,000 • — 54,- 

 000 = 28,000, and a trifle more than that 

 represents thus far in Lyman's work, namely. 



1 ' ' The Electron, etc., ' ' TJniversity of Chicago = ' ' The Spectroscopy of the Extreme Ultra-vio- 



Press, 1917, p'. 202. let," Longman's, 1914, p. 105. 



