DJR. OLIVPm LODGE ON ABERRATION PROBLEMS. 
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called the aberration angle. The velocity with which light journeys over the radius 
vector, r, is 
V cos e + T cos 0, — say : 
the time of the journey being simply t, as before. 
The angle e is defined by the equally obvious geometrical relation 
V sin e — T sin 0 = 0. 
Fisr. t. 
Successive waves emitted by a fixed source S into a drifting medium. Tbe row of dots SG represent the 
respective wave-centres. The figure also rejiresents waves in a stationary medium, emitted by a 
source moving from G to S. 
Here is a picture of the source and successively-emitted and abandoned drilting 
wave-fronts. SM is the path of a labelled disturbance, and is to be considered as a 
ray ; it is inclined at angle e to the corresponding wave-normals. 
SP is what would have been the light journey in the same time if tlie medium had 
been stationary ; PM or SC 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, e, in any direction inclined at angle 6 to the motion, such that 
sin ^ = yr sin 6 = a sin 6, 
it being convenient to denote the ratio of velocities, v/Y, by a single symbol a, 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 
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