RECENT ADVANCES IN SCIENCE 23 



mental data available (to which in collaboration with pupils 

 he has made considerable contributions himself) justifies the 

 assumption that the above equation is one of great generality, 

 embracing " the whole gamut of the radiation scale in its 

 interaction with every type of matter." It is true that in 

 the region of very short wave-lengths (the X-ray region), 

 the maximum energy of electrons liberated by X-rays of 

 frequency n is very closely represented by 



\mv 2 = hn. 



This is consistent with the first equation, since the electrons 

 with maximum velocity will come from the more superficial 

 parts of the atom, and for such electrons w is negligible com- 

 pared with hn in the case of X-rays, as n is so great for such 

 radiation. Prof. Richardson, however, maintains that when 

 we consider all liberated electrons, and not merely those with 

 limiting velocities, the first equation will be found to represent 

 the facts. He goes on in this paper to criticise Prof. Barkla's 

 statement in his Bakerian Lecture (Science Progress, April 

 191 7), that " there is no evidence of absorption of X-radiation 

 in whole quanta, though the conditions are frequently such 

 as give an approximation to this." It is impossible to sum- 

 marise the arguments with sufficient brevity, and they are 

 somewhat obscure in places ; but there is no doubt (as Prof. 

 Barkla himself said in his Lecture) that a closer examination 

 of X-ray phenomena is going to yield the most crucial tests 

 of the generality of the quantum hypothesis, and we may 

 expect in the near future considerable discussion of these 

 matters. 



Resonance radiation forms the subject of one or two 

 papers contributed by Prof. R. W. Wood to recent numbers 

 of the journals. The discovery of the existence of this type 

 of radiation is due to Prof. Wood himself, his earliest inves- 

 tigations being published in 1906, and was made in connection 

 with experiments on the fluorescence of sodium vapour. The 

 absorption spectrum of sodium vapour in a heated condition 

 is very complex, consisting of the principal series of lines 

 (which contains the well-known D lines), and a whole group 

 of fine and sharp absorption lines filling the entire spectrum 

 except a narrow region in the yellow. If the beam of illuminat- 

 ing white light is very powerful, the vapour fluoresces, giving 



