740 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
cross wires, and will be pointing not to the object, but to the centre of the wave it is 
receiving; its collimation axis coincides with a radius or wave-normal, not with a ray. 
(2) A Do])pler alteration of wave-length in every direction; as is obvious in 
the figure, from the distribution of drifted wave-fronts. It is positive on one side, and 
negative on the other side, of a certain direction, 6q, such that the radius vector is 
equal to the radius, or 
CoS — -g- Ci , 
the aberration angle for this particular case of no Doppler effect being twice the com¬ 
plement of 
A spectator moving with the medium will perceive this change of wave-length as a 
change of pitch (or colour) of value 
, n . \ 
lop’ — = loo- — 
^ n ^ \ 
= log (cos e + a cos d)i^ oi cos 0. 
An observer travelling with the medium will not observe any modification in 
interference or diffraction effects, nor will he experience any change of intensity due 
to motion ; for the waves will be brought him at the customary time periods, and be 
subject to the ordinary flux of energy, as if everything were stationary. 
Case of moving Sowce in fxed Medium, 
14. The same figure (fig. 4) serves to illustrate the common case of medium and 
observei' stationary, and source alone moving. 
But we must be careful to note that e is only the aberration angle, and that 
whether it is to be called ‘‘aberration” or not depends on the meaning attached to 
that term. The source emits spherical waves in its successive positions, and leaves 
them to expand at their normal rate. The fixed telescope, pointing to centre of 
advancing wave, is tlierefore pointing to the source at the instant when it emitted 
that light; and, since it is thus seen in its true place at instant of emission, it is 
most natural to say that the aberration caused by moving source alone is nil; for that 
it may have moved by the time of vision, is obvious. 
There is not much more to be said on this head, for the source after throwing oil a 
wave may do what it likes, the light will convey information as to where and how it 
was at the time of emission. Phenomena depending on a succession of waves, e.g., 
changes of pitch, are of course produced, see fig. 4. 
The question arises whether the waves thrown off from a moving source are really 
spherical shells : whether the motion of the source does not affect its vibration ? It 
is not easy to answer this thoroughly and accurately, but practically there can be no 
doubt that the emission of light cannot be affected by any feasible terrestrial motion; 
