754 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
Consequences of Rotational Motion. 
32. If the condition of a velocity potential is not satisfied, it follows from equations 
in §§ 23, 24, that a ray through an ether possessing rotational motion will not be 
straight. 
An ether in the slightest degree carried along in the neighbourhood of moving 
matter must by all analogy be called viscous. Now a ray approaching a moving 
body in a viscous ether will in general be entering strata moving with increasing 
sjDeed, and will accordingly be curved in the direction of the motion (§ 52). A 
negative or lagging real aberration would therefore occur, in addition to the simple 
positive apparent aberration caused by motion of the observer ; and the direction 
actually perceived would be the resultant of the two. The motion of an observer is 
.practically constant all over the earth, but the drift of ether strata would be difierent 
at difierent aspects to the earth’s orbital motion. Hence the observed value of stellar 
aberration ought to vary with the time of day, and with the latitude of the observer.* 
It becomes important to ascertain definitely whether the ether is viscous or not— 
whetheT moving matter can, in the smallest degree, drag or shear the ether in its 
neighbourhood. If it does, either the theory or the observations of astronomical aber¬ 
ration must be overhauled. But experiment is necessary to answer the question. 
Details of Experiment to determine how much, if any, effect on the Velocity of Light 
is exerted hy the Motion of Gross Matter near the Ray. 
33. After considering the motion of belts, of fly-wheels, and of double plates 
rotating oppositely, as in the Holtz machine, I decided to try a pair of plates clamped 
together with a disk-shaped space between them, and to reflect a split beam of 
light several times round in this space, half the light in one direction, and half in 
the other, wliile the pair of plates were revolving at a high speed. Michelsox’s 
device for obtaining two equal beams of light travelling in opposite directions rouiid 
a contour, by means of a semi-transparent mirror, is plainly the most suitable for any 
case where the effect of motion is to be observed, and where great length of path is 
desirable. 
Accordingly I sent to Mr. Adam Hilger a strongly braced stout wooden frame, or 
hollow square, 38 inches in the side, internal measurement, and asked him to fit it— 
(J) with three plane mirrors, each 6X2 inches, supported in a specified fashion, and 
silvered on the front; (2) with a fourth mirror, 4X2 inches, supported rather 
differently, bevelled to 45° at two of its edges, and likewise fully silvered on the 
[* It lias been pointed ont also by Professor Fitzgerald tbat, if snob stratified motion existed, the 
top of a tower or mountain should exhibit aberrational effects when viewed from below. This might be 
tested with greater accuracy than is possible in celestial observations.—July, 1893.] 
