called the aberration angle. The velocity with which light journeys over the radius vector, , is
, say:
the time of the journey being simply , as before.
The angle is defined by the equally obvious geometrical relation
.
Fig. 4.
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Successive waves emitted by a fixed source into a drifting medium. The row of dots represent the respective wave-centres. The figure also represents waves in a stationary medium, emitted by a source moving from to
Here is a picture of the source and successively-emitted and abandoned drifting wave-fronts. is the path of a labelled disturbance, and is to be considered as a ray; it is inclined at angle to the corresponding wave-normals.
is what would have been the light journey in the same time if the medium had been stationary; or represents the drift.
The result of the state of things exhibited in the diagram may or may not be appreciated by a spectator—that depends on what his own motion is,—but if he is moving simply with the medium, he perceives the following:—
(1) An aberration, , in any direction inclined at angle to the motion, such that
,
it being convenient to denote the ratio of velocities, , by a single symbol , and to call it the aberration constant. A telescope moving with the medium and placed with its object glass tangential to the advancing wave-fronts, will focus the image on its