796 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
incident and reflected rays AO and OB, incident at the point 0, affected by the drift 
in such a way that their direction is A'O and OB' respectively ; but they are 
still both in a plane normal to the surface, and they still make equal angles with the 
surface. 
Fig. 16. 
A' 
And as regai'ds refraction, an equal length of the refracted ray wflll be foreshortened 
into OC = OB/p,; and the drift inside the medium being, according to Fresnel’s 
theory, BB'/p-, will in the longer time available carry C to O', such that OC'B' 
remains a straight line; accordingly, the plane of the refracted ray is still the plane 
of incidence. Hence the perpendicular component of the drift is wholly inefiective, 
and vce have only to consider the component in the plane of incidence ; call it v. 
Component of Drift in Plane of Incidence. 
Consider a plane wave AqBq travelling, with an aberration angle e between its 
normal and its direction of advance caused by a drift in direction AA', to’wards a plane 
mirror, at a pace compounded of the speed of light and of the drift, viz., 
Y cos e + V cos 6. 
Let incident and reflected rays make angles i and i' respectively with the mirror- 
normal, and 6 and 6' witli the line of drift, so that if (f) is the inclination of the 
drift direction to the mirror-normal, 6 = i — (f), tt — 6' — i' (f). 
Pig. 17. 
The incident and reflected waves will be inclined to the surface by the angles ^ fl- e, 
i' — e, respectively. 
When the wave AB strikes the mirror, A becomes a source of radiation, and B 
travels on to C, with a velocity compounded of BB' and B'C. By the time B arrives 
at C the radiation from A will have spread a distance equal to B'C through the 
medium, and the centre of a wave with this radius will have had time to drift with 
the moving nfedium to A', a distance equal to BB'; hence, drawing the semicircle 
A'B'EC, and choosing on it a point E, such that A'E = B'C, we have tlie direction 
of the reflected ray, viz., AE, and of the reflected wave CE. 
