782 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
telescope will be independent of the medium inside it, although the course of a rav 
through it is really altered [viz., from AB to AE. The aberration angle in vacuo is 
MAB ; in the medium is A"EO. The diminished velocity of light is exactly com¬ 
pensated by a virtually diminished speed of telescope with respect to the ether inside 
it; and so a steadily moving telescope sighted on a star can remain parallel to itself, 
with v'hatever medium it may be filled.—July, 1893.] 
52 But, by moving a portion of medium relatively to the observer, say by spinning 
a glass disk, and looking through it axially near its circumference, where its velocity 
is u, we are looking through a moving stratum of thickness 2 , and a parallel shift may 
in that case be experienced. 
The angle is 
_v _ (I - 
y I/Ji A /yU. 
or for glass about 
fMlC 
5 
and the shift is 2 times this angle, viz., 
IXZU 
* = 2 ?- 
To give a shift of I micron = a thousandth of a millimetre = 10”^ centims., with a thickness 
2 : = 10 centims., would require a velocity u — 2Ysj/nz = (4 X 10^® x I0”^)/I0 == 4 x 10® centims. pei 
second = 4 kiloms. per second. My machine, at 3000 revolutions a minute, 50 a second, gives a 
periphci’al speed of 50 x 3 = 150 metres per second, so the thickness of glass needed to give a shift of 
1 micron is 2 : = 2Vsl/iu = (4 x I0^° x 10~‘*)/15000 = 4000/15 = 3 metres. To-and-fro reflexion may 
he used to diminish the required thickness. 
More detailed discussion of Doppler Effects. 
53. There is rather a nice point to be considered in connexion with change (2), §9, 
viz., what the pitch, as perceived, really depends on. The coarse statement of examina¬ 
tion candidates that it depends on wave-length, or on the frequency of vibration of the 
source, is of course not true; it depends on the frequenc}^ of disturbance reaching the 
receiver. This fact is suggested by listening or looking through a difierent medium, 
wherein the wave-length is quite different; though, indeed, it must be admitted that 
the medium in ear or eye cannot be changed. It is proved (for the case of sound at 
