and Stoke s- Planck' s JEther. 169 



Such, then, would be the required refractivity of the 

 -condensed aether, obeying any law p=f(p). In particular, 

 if it obeys Boyle's law, we have 



n 2 = 1 + 4- 2 .logs, .... (10a) 



which is of a surprisingly simple form, and reads: n 2 — 1 

 •equal to four time* the logarithm of condensation multiplied bg 

 the squared ratio of the two velocities of propagation charac- 

 terizing the cether. 



Notwithstanding this temptingly simple form of the 

 relation, I shall not try to "deduce" it from things more 

 familiar. I prefer to regard it as an assumption, dictated by 

 observation. 



If the reader so desires, he can write n 2 — l = 4icjc 2 } where 

 iv is the work, per unit mass of matter, done by the gravita- 

 tional field in condensing the aether. The small fraction 

 n 2 — 1 being known from the Eclipse results (for any r), the 

 numerical value of this work is determined without any 

 further assumptions. If we agree to the lowest estimate of 

 log s at the Sun's surface, as required by the aberration 

 theory, we can also evaluate separately the ratio v/c, as 

 already mentioned. This, however, is only a secondary 

 matter. 



7. Some details and further implications of the Stokes- 

 Planck aether theory, supplemented by assumption (10), 

 must be postponed to a later opportunity. Here it will be 

 enough to add only a few more general remarks. It will be 

 kept in mind that the proposed theory would account not only 

 for the observed astronomical aberration and for the older 

 terrestrial optical nil-effects, but manifestly also for the nil- 

 effect of the Michelson-Morley experiment. The bending of 

 rays round the more massive celestial bodies would be only a 

 by-product of the theory. Again, in view of the exceedingly 

 small condensation of the aether round single atoms or cor- 

 puscles there will be no difficulty in working out a satisfactory 

 •electromagnetic theory of ponderable media. The proposed 

 theory would also have the advantage of not predicting the 

 obstinately absent gravitational shift of the spectrum lines. 

 It might also react, in part at least, upon the 1905 relativity, 

 depriving it of its indispensability in most cases, but by no 

 means banishing it from the whole domain of physico- 

 mathematical investigations. Finally, the just objections 

 raised by the advocates of the physical principle of causality 

 ugainst the fixed and homogeneous aether of Fresnel-Lorentz 

 would not apply to Stokes's modified aether. For this 



