544 PROFESSOR J. H. POYNTIXG ON RADIATION IN THE SOLAR SYSTEM: 
carrying properties. Whether the momentum in the medium is in the form of mass 
in moving with velocity v in the direction of propogation is perhaps open to doubt. 
We may, perhaps, have different forms of momentum just as we may have different 
forms of energy, and possibly we ought not to separate the momentum in radiation 
into the factors m and r, but keep it for the present as one quantity M. 
An interesting example of inequality of the radiation forces on two mutuallv 
radiating bodies is afforded by tw-o equal spheres, for w4iich, at a given temperature, 
the radiation push F balances the gravitation pull P. Raise one in temperature so 
that the push on tlie other becomes F'. Lower the other so that the push on the 
first becomes F", but adjust so that 
tlien 
F + F" = 2F = 2P, 
P — F" = F' - P. 
There will then l)e e»pial accelerations of the twm in the .same, not in opposite 
directions, and a chase will begin in the line joining the centres, the hotter cliasing 
tlie colder. If the two temperatures could be maintained, the velocity would go on 
increasing ; but the increase w-ould not be indefinitely great, inasmuch as a Doppler 
eftect would come into jday. Each sphere moving forward would crowd up against 
the radiation it emitted in front, and open out from the radiation it emitted back¬ 
wards. This would increase the front and decrease the back pressure, and ultimately 
the excess of front pressure would Ijalance the accelerating force due to mutual 
radiation. 
Let us examine the efiect of motion of a radiating sui-face on the pressure of its 
radiation against it. 
Application of Doppler’s Principle to the. Radiation Pressure against 
a Moving Surface. 
If a unit area A, fig. 4, is moving with velocity u i]i aii}^ direction AB, making 
angle x}j with its normal AN, the effect on the energy density in the stream of 
radiation i.ssuing in any direction AP is two-fold. If the 
motion is such as to .shorten AP, the waves and their 
energy are crowded iqD into less space, and if such as 
to lengthen AP, they are opened out. At the same time, 
in the one case A is doino’ work ao’ainst the radiation 
o a 
pressure and in tiie other is having work done on it. We 
.shall assume, as in the thermodynamic theoiy of radiation, 
that this work adds to or subtracts from the energy of 
radiation. Both effects (l) the crowling, and (2) the 
work done, or the reverse of each, combine to alter the energy and therefore the 
radiation pressure. We have no data Iw vinch we can determine wdiether the 
