802 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
Now whatever the initial lag may be, and it may be arbitrary, the final lag will 
differ from it by this same amount; and if the rays, instead of being parallel, are 
coincident in path, then no difference in phase is caused by reflexion. 
Change of Energy at Reflexion from Moving Mirror 
74, When reflexion takes place from a moving (receding) mirror, there is some work 
done on the mirror by reason of the intrinsic pressure of light. Calling the energy 
per unit volume e -f e, the energy of the incident light per second is Ve, and of the 
reflected Ve', the pressure is e + e, and the work done 23er second (e + e) v. 
So 
Ye - Ye' = (e + e) v, 
or 
e (1 — a.) = e {1 + a). 
Or consider the mirror fixed and medium moving with source away from it, 
the speed of incident light is Y — v, of the reflected is Y + v, but no work is clone; 
so 
(Y — v) e = (Y + v) e . 
Wherefore, on either mode of consideration, the energy of the reflected light is from a 
receding mirror less, from an advancing mirror greater, than that of the incident 
light, in the ratio 
d _ 1 - « 
6 1 + « 
Possible Effect.of Light Pressure in Astronomy. 
75. Light energy per unit volume on Mount Whitney, as determined by Langley, 
amounted to 67 microbarads, or, say, in outside space, three quarters of 10“"^ ergs per 
<3ubic centimetre; giving a pressure of the same number of dynes per sq. centim. 
This pressure on the Moon is wltlidrawn during eclipses, but, although equal to 
the ordinary weight of 10,000 tons or so, it is too small to make sensible perturbations, 
as it could only push the Moon ^^th inch in a fortnight. 
On a small body, however, it may become comparable with gravitation."^ 
On a small-enough dust particle, such as may be in tails of comets, the light 
pressure and gravitative attraction of the Sun might balance. I make the size about 
1 micron diameter for a sphere of the density of water, at any distance. Anything 
smaller than this would be repelled, and Avould get up an excessive velocity in time. 
[* I find that Fitzgerald made a communication years ago to the Royal Dublin Society on this 
subject.] 
