738 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
the idea beino’ that with a finite speed of propagation the lines of foi’ce would lag, and thereby acquire a 
curvature out of the magnet’s meridian ; so that a distant needle instead of pointing straight at the 
magnet would be tangential to these lines, and would therefore be slightly deflected during the spin. 
We now see, however, that no such aberrational effect is to be expected, except on a corpuscular view 
of magnetic pi’opagation. 
Concerning the eftect of motion of other kinds, certain things are experimentally 
known; e.g., motion of the receiver is known to cause aberration, however the fact 
be precisely accounted for; and motion of the medium alone is known not to cause 
aberration of any perceptible magnitude, else would terrestrial surveying operations 
be inaccurate. But no experimental data as yet obtained are evidence concerning 
small quantities of the second order, and it will be well to examine critically and 
geometrically the whole subject of wave motion from a moving point to a moving 
telescope through a uniformly moving medium, all the velocities being possibly 
different in magnitude and direction. So far as steady and uniform motion is 
concerned this may be considered the most general case. 
Convenience of attrihuting Relative Motion to Medium. 
12. Before considering separately the phenomena mentioned in §9, it may be con¬ 
venient to consider what it is which must be in motion in order to produce one or 
other of them. And, first, which of them a motion of the medium alone causes. 
Nothing can be more certain than that relative motion is all we are concerned with, 
so that whether a source travels through a medium, or the medium drifts past the 
source, comes to precisely the same thing. Sometimes one mode of expression is con¬ 
venient, sometimes the other. It may be most natural to contemplate the medium 
as stationary, and to throw all motion on source and receiver, but I find that it is 
often very simple and helpful to invert this order, and to think of the ether of space 
as drifting past the earth, or other body, supposed stationary. 
We shall not invariably use this device, but whenever a number of things—source, 
mirrors, telescope, and observer—have to he thought of as moving all precisely alike 
through the ether, it is simpler to think of the ether as streaming past them. 
Case of Fixed Source in Moving Medium. 
13. Consider now a fixed point-source in a uniformly moving medium. • 
Spherical wave-fronts are thrown off and immediately begin to drift, so that their 
centres get displaced a distance, vt, while their radii enlarge by an amount, ; and 
the distance through space which a disturbance has by that time travelled in the 
direction 9 will be compounded of these two distances, and will be inclined to 
the radius, or direction of travel if all were .stationary, by an angle e, which may be 
