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. Fordescription of this experiment 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, the possible phenomena caused by motion are changes in direction, in period, in phase, and in amplitude.
Apparent Direction as Affected by 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 direction (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, with the target moving across the line of fire, the gun must
