358 SCIENCE PROGRESS 



close electrodes (o'l to 2 mm. apart) with a battery of Leyden 

 jars charged to a potential of several hundred thousand volts 

 by a powerful induction coil. For the maintenance of this 

 type of spark the pressure must be kept below 10"* mm. : this 

 was secured by using a mercury diffusion pump, kept in con- 

 tinuous operation. The shortest wave-lengths thus far obtained 

 in the spectra of carbon, zinc, iron, silver, and nickel are 360-5, 

 3 1 7*3, 2 1 'j'6, 260, and 202 angstroms respectively. The spectrum 

 of carbon was found to be particularly brilliant, more than 

 fifty bright lines with wave-lengths shorter than i ,000 angstroms 

 being obtained ; although the other metallic spectra obtained 

 showed a greater number of lines, they were for the most part 

 weak and somewhat diffuse. It seems to be certain that the 

 intense spark employed causes the carbon to volatilise, and 

 that the lines obtained are true gas lines. Prof. Millikan 

 advances reasons for believing that the lines obtained belong to 

 the L-series of the X-ray spectrum of carbon. In favour of this 

 supposition is the fact that the spectrum stops abruptly at a 

 wave-length of 360*5 angstroms, whilst lines of shorter wave- 

 length were obtained with the other elements examined, and 

 this limit is about the wave-length at which the L spectrum of 

 carbon might be expected to commence, if extrapolation is 

 justifiable from Moseley's measurements on the L rays of 

 elements having atomic numbers from 30 to 90. Additional 

 facts in support of this conclusion are (i) that the relative 

 wave-lengths and intensities of the three shortest lines in the 

 spectrum are closely similar to those shown by the three main 

 lines of the L-series ; (2) that the strong line corresponding to 

 La in carbon is double, and it is well known that the alpha 

 line of the L-series is a double in all elem^ents. 



This conclusion, if justified as it appears to be, is of great 

 importance, because no X-ray spectra of the L-series have 

 ever been taken with crystal gratings in the case of elements 

 of atomic number less than 30 ; and for so low an atomic 

 number as carbon the grating-space of a crystal grating would 

 be too small in comparison with the wave-length for sharp 

 images to be obtained. Further, on this hypothesis, no other 

 lines in the carbon spectrum are to be expected until the 

 K-series is reached, the longest wave-length of which should be 

 about 45 angstroms. At present, therefore. Prof. Millikan 

 has not only very considerably extended our knowledge of 

 spectra in the extreme ultra-violet region, but he has also 

 nearly succeeded in bridging the gap between the visible 

 spectrum and the X-rays of shortest frequency. 



Stellar Distribution and Motions. — One of the most im- 

 portant problems of stellar statistics is the determination of the 

 laws according to which the stars are distributed throughout 



