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 is carried down stream in undue proportion towards , the amount actually emitted in this direction is diminished in a compensatory manner, so that the resultant flux of energy remains unaffected 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 radiates less than when the medium is stationary, a surface like 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 temperature 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 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 as if it were stationary and the medium drifting (fig. 4), and the eyepiece 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 pointing straight at a source (fig. 6), and at the instant a given luminous disturbance starts from , let the telescope begin moving in a direction with a velocity . Let it thus reach the position by the time the light has got as far as , i.e., to the spherical wave front indicated in the diagram. Then it follows that by the time the telescope has reached the position the light will have reached , too, and will accordingly have passed along the collimating axis by reason of the combined motions.
