Prof. J. J. Thomson on ilie Cathode Rays. 415 



The thickness is in millionths of a millimetre, i. e. is in terms 

 of the milli-microm called by microscopists p/i. 



The films were on glass, and the absorption of the glass was 

 allowed for by control experiments. 



It is to be understood that of the whole incident light the pro- 

 portion reflected is first subtracted, and the residue is then called 

 1 in order to reckon the fraction transmitted of that which enters 

 the metal, it being understood that the residue which is not trans- 

 mitted (say *68 or *63 in the case of platinum) is absorbed. It 

 may be that more and better work has been done on the opacity of 

 metals than this : at any rate there seems to me room for it. I 

 do not quote these figures with a strong feeling of confidence in their 

 accuracy. They are to be found in Wied. Ann. vol. xxxv. p. 57. 



XXXVI. Note on Mr. Sutherland's Paper on the Cathode Rays. 



To the Editors of the Philosophical Magazine. 

 Gentlemen, 



IN the March number of the Philosophical Magazine 

 Mr. Sutherland considers a theory of the cathode rays- 

 which I published in this Journal in October 1897, and in 

 which the carriers of the charges were supposed to be the 

 small corpuscles of which the atoms of the elementary bodies 

 could, on an extension of Prout's hypothesis, be supposed to 

 be built up. Mr. Sutherland takes the view that in the 

 cathode rays we have disembodied electric charges, charges 

 without matter — electrons — their apparent mass being due to 

 the energy due to the magnetic force in the field around them 

 varying as the square of the velocity (see Phil. Mag. xl. p. 229). 

 I may say that the view that in the cathode rays the con- 

 stancy of the mass arose from the charge being torn away 

 from the atom, so that we had only the effective mass due to 

 charge, occurred to me early in my experiments, but except 

 in the form (which I gather Mr. Sutherland does not adopt), 

 and which only differs verbally from the view I took, that the 

 atoms are themselves a collection of electrons, that is, consti- 

 tute an assemblage of particles the individuals of which are 

 the same as the carriers in the cathode rays ; this conception 

 seemed to me to be wanting in clearness and precision, and 

 beset with difficulties from which the other was free. In 

 the theory which I gather Mr. Sutherland holds of the 

 cathode rays, we have atoms which are comparatively large 

 systems ; these can be charged with electricity, of which in 

 electrons and neutrons we have what correspond to atoms 

 and molecules, the radius of an electron being about 10~ u cm. 

 What conception must we form of the connexion between the 



