1921-22.] The Faraday-Tube Theory of Electro-Magnetism. 241 
indirectly — suggested, I should like to ask your advice about it. You may 
remember recommending to your classes in Watson’s some years ago 
Dr Norman Campbell’s little book on Electricity in the People’s Library 
Series. ... I worked more or less on the lines of the bibliography in 
Dr Campbell’s book, at least at first, but I was always curious to learn 
more about the theory of Faraday -tubes which he sketches. I eventually 
read the original investigations in Sir J. J. Thomson’s Recent Advances, 
but could not understand his proofs in the light of Dr Campbell’s ideas 
(and also those of Heaviside and Maxwell). When I referred at last to 
Dr Campbell’s Modern Theory of Electricity I was disappointed to find 
him refer readers for proofs to J. J. Thomson’s work. The more I began 
to understand the ideas of the Recent Advances proofs, the less inclined 
was I to admit their justification for Dr Campbell’s purposes. I came to 
the conclusion that the Faraday- tube theory, which perhaps Thomson 
regarded more as an aid to thought than as a serious physical hypothesis, 
is too good for such a subordinate place as a mere easy guide to Maxwell ; 
but it had not yet been sufficiently developed in a consistent manner so as 
to take rank as a physical theory. 
“ The paper I have written, after struggling with these difficulties for 
about two years, proposes to supply this development. It is written mostly 
in Heaviside’s vector notation. . . . New proofs directly from dynamical 
hypotheses are given of three of the five laws of electro-magnetism. There 
is also a good deal of other matter in the paper . . . for instance, a 
modified theory of stresses, and an extension of Campbell’s discussion of 
propagation so as to include an explanation of aberration, etc. It was 
writing this section which suggested my note in the Phil. Mag. of last 
August. . . . 
If you and Professor Gray mean to communicate my note on ‘ Mass as 
a Linear and Vector Operator ’ to some society, I could send you a slightly 
fuller typewritten copy ; but I am afraid it is not up to R.S.E. standard 
at any rate. 
“ I am now on convalescent leave after having dysentery at Gallipoli, so 
have a good deal of time on my hands. 
“ Hoping you are keeping well during the winter, — Your affectionate 
pupil, W. G. Brown.” 
The note here referred to on mass as a linear vector operator will be 
discussed more fully below. 
Meanwhile W. G. Brown had been grappling with the Theory of 
Relativity, as is shown both in his letters to his parents and in the 
VOL. XLII. 16 
