750 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
function of their direction of motion through the ether ; and accordingly that the 
length and breadth of Mtchelson’s stone supporting block were differently affected, 
in what happened to be, either accidentally or for some unknown reason, a compen¬ 
satory manner. 
26. There is already one experiment, which I have never seen criticised either way, 
tending in a sense precisely contrary to Michelson’s. Fizeau* observed the polari¬ 
zation produced by a pile of plates, and considered that he had proved that the 
azimuth of the plane of polarization varied with the direction of orbital motion of the 
Earth, and hence that the ether was streaming past them. If so, polarization by 
reflexion is the only phenomenon known which is capable of showing a first-order 
effect of the general ethereal drift. The experiment seems to me extremely difficult, 
but to be well worthy of repetition by other observers. [I believe that Lord 
PtAYLEiGH’s objection to the experiment as performed by Fizeau is that the effect 
was unseen until an illegitimate or unsafe magnifying device was employed.] 
Meanwhile I shall hope to examine the question of ether motion near moving matter 
in a simpler fashion (§ 33). 
Assuming for the present that the ether is not disturbed in a viscous manner by 
the motion of gross matter through it, we can make the following assertions :— 
General Statements Concerning Aberration. 
27. A my is straight whatever the motion of the medium, unless there are eddies, 
and accordingly no irrotational currents of ether can divert a ray. But, if the 
observer is moving, the apparent ray will not be the true ray, and accordingly the 
line of vision will not be the true direction of object. 
In a stationary ether, wave-normal and ray coincide, but the line of vision of a 
moving observer slants across both (§ 20). 
In a moving ether, wave-normal and ray enclose an angle, and line of vision 
depends upon motion of observer. If the observer is stationary his line of vision is 
the ray ; if he moves at the same rate as the ether his line of vision is the wave- 
normal (§ 13). 
The line of vision, in fact, always depends on the motion of the observer, not at all 
on the motion of the ether so long as it has a velocity-potential. Hence nothing can 
be simpler than the theoiy of aberration if this condition is satisfied. 
A similar but more general condition (to be obtained in the next section) suffices 
to secure the straightness of a ray whatever happens, or more generally that whatever 
the path of a ray may be by reason of reflexion or refraction in a stationary ether, 
the same it sliall be in a moving one; and readily accounts for the absence of all 
effect on direction due to the general relative drift of the medium, whether in the 
* ‘Anil, de Cliim. et de Pliys.,’ 1859, vol. 57, p. 129. 
