i2 4 SCIENCE PROGRESS 



hardest which we can produce down to that characteristic of 

 aluminium on the one hand, and by using visible and ultra- 

 violet light on the other. He further shows that the existence 

 of rings intermediate between the outermost and the K and L 

 rings must be detected and their properties studied by the use 

 of radiation of wave-length between that of the K and L 

 radiations and that of the highest ultra-violet yet obtained, that 

 is radiation in a gap of about eight octaves between 36 x io -8 cm. 

 and 9 x io~ G cm. This paper then proceeds to describe attempts 

 which he has made to produce such extremely soft Rontgen 

 radiation by the impact of positive rays and slow cathode rays 

 on a platinum plate. After taking all possible precautions to 

 eliminate spurious effects, he has succeeded in detecting such 

 radiation by its action on a photographic plate. He finds that 

 even the thinnest films of such substances as collodion, mica, 

 wax, aluminium are opaque to it, and he suggests how the 

 ordinary theory of dispersion may be adapted to explain this 

 opacity ; that, in fact, an atom will scatter strongly, and there- 

 fore be impervious to, rays whose wave-lengths lie between 

 limits which depend upon the density with which the electrons 

 are packed within the atom, and the natural periods of these 

 electrons. 



Two very interesting letters from Prof. Barkla to Nature 

 (February 18 and March 4, 191 5) deal with the relations of 

 X-radiation and corpuscular radiation to the K and L rings. 

 Prof. Barkla supplies experimental evidence favouring Bohr's 

 theory, and states that further experiments are in progress 

 to elucidate some points in connection with the ejection of 

 electrons from the atom produced by those disturbances of the 

 rings which also produce the radiation. 



Accounts have been published recently of experimental work 

 connected with series spectra which is crucial as regards the 

 validity of Bohr's Theory. According to Bohr's Theory, certain 

 spectral lines which have generally been attributed to hydrogen 

 ought to be attributed to helium, and on his view also certain 

 other lines previously unobserved should make their appearance 

 in the spectrum of helium. In the February Phil. Mag. there is 

 an account of some experiments by Mr. Evans which to some 

 extent justifies these conclusions. Mr. Merton, however, in a 

 letter to Nature, March 18, combats this view, and while main- 

 taining that Mr. Evans's experimental work is not really 



