DR. OLIVER LODGE OR ABERRATION PROBLEMS. 
745 
between them, or a pair of equally hot bodies with a thermopile half-way between 
them, all subject to an ethereal drift in the direction of the arrow, we may assert 
that although the radiation from A is carried down stream in undue proportion 
towards C, the amount actually emitted in this direction is diminished in a compensa¬ 
tory manner, so that the resultant flux of energy remains unaflected by the motion. 
It is not necessary to suppose that motion disturbs the equality which otherwise 
exists between radiating and absorbing powers. It is true that if a surface like A 
radiates less than when the medium is stationary, a surface like C facing the stream 
must radiate more ; but then it may absorb more also. So that in all respects the 
balance may be undisturbed by the motion of the medium. 
It is probable, therefore, that even by this intensity method, nothing more than the 
second order of aberration magnitude is effective for displaying a general drift of the 
medium as a whole. 
At the same time it seems desirable that an experiment with thermopiles, like that 
suggested by Fizeau, should be tried, in order to verify the above deductions from 
the theory of exchanges, combined with the supposed persistent uniformity of tempera¬ 
ture of an enclosure whether at rest or in motion ; for thereby the absence of friction 
or dissipation of energy by motion of solids through ether would be verified. 
Case of only Receiver Moving. 
20. If the receiver be not fixed relatively to the medium, nor relatively to the 
source, but be moving on its own account, the effects due to this motion must be 
added to the preceding effects. First suppose both source and medium stationary. 
The source S emits waves in spherical 'shells, whose radii are also rays. Any 
motion of the receiving telescope can be resolved tangentially and radially. Radial 
motion gives Doppler effect only; tangential motion gives aberration only—both of the 
commonplace type. 
If the telescope were stationary, its object-glass must be tangential to the wave 
front, but directly it moves it must encounter the wave front obliquely, with the same 
obliquity e as if it were stationary and the medium drifting (fig. 4), and the eye¬ 
piece will then be brought to the light at the right instant. Revolution of a radial 
telescope about the source would effect this in the simplest way, without introducing 
any Doppler effect or change in focal length. 
Consider a telescope OqEq pointing straight at a source S (fig. 6), and at the instant 
a given luminous disturbance starts from S, let the telescope begin moving in a 
direction (f) with a velocity u. Let it thus reach the position OE by the time the 
light has got as far as O, i.e., to the spherical wave front indicated in the diagram. 
Then it follows that by the time the telescope has reached the position OjEj^ the light 
will have reached E^, too, and will accordingly have passed along the collimating axis 
by reason of the combined motions. 
MDCCCXCIIT.—A. 5 C 
