DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
735 
The two parts of Fresnel’s law, the motion of internal ether, and the fixity of 
external ether, can and ought to be verified separately. The Fizeau experiment has 
verified the one. I propose to attempt the other. To this end I am passing a beam 
of light, split into two equal halves, very near a rapidly rotating disk (in fact between 
a pair of rotating disks clamped together), so that one half the light travels with the 
mechanical motion and the other half travels against it. The two half beams, after 
several journeys round and round, are united, with interference effects, and the 
observation consists in watching the system of bands for any shift caused by the 
motion. For description of this expei’iment see §§ 33-47 below.' 
Phenomena Resulting from Motion of Source, Receiver, or Medium. 
9. The phenomena which can be appealed to as evidence of a state of motion, and 
which necessarily result from that motion if of a suitable kind, are four, viz.;— 
(1) Changes or apparent changes in direction of ray, as observed by telescope 
with cross-wires ; the change commonly called “ aberration ” proper. 
(2) Changes or apparent changes in frequency of vibration, as observed by the 
pitch or colour appreciated by an observer, or by the shifted position of lines 
in a spectroscope; a change which may be referred to as the Doppler effect. 
(3) Changes or apparent changes in the time taken over a fixed journey, as 
observed by the relative lag in phase between two portions of a split beam 
and the consequent shift of interference fringes when they are re-united. 
(4) Changes or apparent changes in the intensity of radiation in different 
directions, as observed by the amount of energy received by a given area 
exposed normal to the rays at a given distance from a source, but having 
different aspects with respect to the line of motion. 
Or, briefly summarizing them, thei possible phenomena caused by motion are changes 
in direction, in period, in phase, and in amplitude. 
Apparent Direction as Affected hy Motion in General. 
10. Consider the subject first from a corpuscular or projectile point of view, first 
ignoring the medium. A gun travelling broadside on must be aimed behind the 
object, and its shot will travel in a skew dmection (keeping always straight in front of 
the muzzle, but not travelling along the axis of the gun) with a velocity compounded 
of the speed of projection and the speed of the gun. The apparent position of the 
source, as recognized through a hole in the target, will therefore be its true position 
at time of firing, but not its position at time of hit. 
Whether we choose to call this an aberration or not is a matter of nomenclature 
merely. 
If the gun is fixed, witli the target moving across the line of fire, the gun must 
