801 
DE. OLIVER LODGE ON ABERRATION PROBLEMS. 
vations of HoEKand others, performed with terrestrial light, aimed only at disproving 
Klinkeefues’ notion that an error proportional to the hrst power was to be expected, 
and did not aim at the immense delicacy needed to observe ah 
Wave-length as altered hy Reflexion. 
72. Since the laws of reflexion are so closely obeyed, an image in a mirror will prac¬ 
tically appear just the same whether the medium is stationary or not, and, accord- 
ingly, the image may be treated as the virtual source for all cpiestions relating to 
wave-length and Doppler effect, and the waves coming from that image will in general 
be affected by tlie drift otherwise than are the weaves coming from the source, because 
the direction is different. 
For instance, sunshine strikes the earth perpendicularly to its motion, but reflected 
sunshine may coincide with the direction of motion, and, in that case, will have t(^ 
travel against (or with) tlie ether wind precisely as if it came from a terrestrial 
source, and its wave-length will be affected as already reckoned ; in other words, 
thinking of a mirror moving with the orbital motion of the earth only, considered as 
circular, the image of the Sun moves as if attached to the mirror (not at twice the 
rate), and, accordingly, reflected sunshine behaves as regards w^ave-length precisely 
as if it were coming from a terrestrial source. [More generally {i.e., including 
eccentricity of orbit and aberration) reflected light as seen hy an observer moving ivith 
the mirror appears in every respect like direct light.] 
For irregular reflexion, e.g., from white paper or from the Moon or a planet, these 
things can be treated as being themselves the sources. 
Change of Phase caused hy Reflexion in a Moving Medium. 
73. Now consider the phase as affected by reflexion. 
Consider the two parallel rays A and B, in fig. 17, distant h from each other, and 
let B lag initially by an amount h tan e behind A (§67), then, after I’eflexion, the 
distance apart has changed so that 6/cos e = h'jcos e = c say, and the lag is h' tan e. 
Hence the gain of lag by reflexion, h' tan c — h tan e 
= c (sin e' — sin e), 
i + iW / , i-i'\ 
= — 2ac cos —-— sm ifj) -> 
which, very approximately, 
= — 2ac cos i sin 
For normal incidence and tangential drift it has its maximum value, 2a6. 
MDCCCXCIII.—A. 5 K 
