586 Prof. J. A. Fleming on the Measurement 
force above, as acting on the interfacial current-sheet, it is 
not difficult to verify that when the incidence is oblique, the 
incident, reflected, and refracted wave-trains exert indepen- 
dently on the reflecting surface their full oblique thrusts in 
their own directions of propagation, as is implied in Prof. 
Poynting’s calculations referred to at the beginning. 
The result here verified, that motion of a material body 
does not affect the pressure exerted on it by the ambient 
radiation, has been rejected by Prof. Poynting in a later 
postscript added to the memoir above referred to, on the ground 
that radiation shot out of a radiator A into a moving absorber 
B would, according to it, alter the store of momentum of the 
two bodies. But if the bodies are in thermal equilibrium, 
other compensating events are at the same time occurring, viz. 
the absorber B is also radiating towards A. And indeed if 
the temperatures of A and B are unequal, the aggregate 
momentum of both admittedly does change on account of their 
radiation. 
If the present argument is right, the view which considers 
a ray to bea simple carrier of momentum from the one body 
to the other cannot therefore be maintained. 
It may be noticed, in connexion with p. 584 supra, that 
for the same amplitude of ionic excursions in the vibrating 
molecule, as determined by its maximum electric moment, 
and for the same periodic time, it follows from Hertz’s 
formule for a simple radiator, and may be generalized by 
the theory of dimensions, that the radiation emitted per 
unit time is proportional to the refractive index of the 
surrounding medium, and therefore the equilibrium-density 
of the radiation in that medium is proportional to the square 
of the same index, in accordance with Balfour Stewart’s law 
derived from the doctrine of equilibrium of exchanges between 
sources at uniform temperature. | 

LXIV. Note on the Measurement of Small Inductances and 
Capacities, and on a Standard of Small Inductance. By 
J. A. Fremine, D.Sc. F.R.S., Professor of Electrical 
Engineering in University College, London*. 


‘To AST year a paper was read before the Physical Society 
by the present writer and Mr. W. C. Clinton, on the 
‘¢ Measurement of Small Capacities and Inductances” f. 
In that paper we described two forms of motor-driven 
commutator for the measurement of small capacities and 
* Communicated by the Physical Society : read March 25, 1904. 
+ See Proc. Phys. Soc. Lond. vol. xviii. p. 386; also Phil. Mag. May 
1903, p. 493. 
