362 
POPULAR SCIENCE REVIEW. 
which seems, on the face of it, as hopeless as that of determin- 
ing what the stars are made of would have appeared' twenty or 
thirty years ago. 
Yet the problem has been mastered, and in a much more 
thorough manner than the seemingly simple problem of esti- 
mating the motion of the stars athivart the line of sight. In 
fact, whereas we know nothing (except in one or two cases) 
respecting the amount of this latter motion, for its angular 
measure affords no satisfactory criterion of the real motion in 
miles per year, we have a means of measuring the real velocity 
with which the stars are approaching towards us or receding 
from us. I proceed to show how this is done : — 
We know that light is not a material emanation from lumi- 
nous bodies, but consists in reality of the propagation of 
minute vibrations, taking place either in an aetherial medium 
pervading all space, or (as some physicists suppose) in the 
ultimate particles of what we ordinarily term matter.* Now 
it is to this property of light that the powerful mode of research 
which I am about to describe owes its effectiveness ; and there- 
fore it is necessary that we should attend somewhat closely to 
the peculiarities of wave-motion. Space will not permit me to 
enter at such length as I could wish into the considerations 
thus arising ; and therefore I will confine my attention to a few 
primary points. The whole subject was dealt with in a highly 
interesting manner, I may remark, in the lecture delivered by 
Professor Miller to the working-men of Exeter [see p. 335], 
the careful study of which will well repay the student. 
• It is often difficult in treating of the supposed fetlierial medium to ex- 
press oneself at once intelligibly and accurately. If there is an a*therial 
medium difierent in some essential properties from any of the substances 
we recognise as matter, yet that aether is still matter, and the laws which 
rule its movements are identical with those which govern the movements of 
other matter. By the term laws ” I here signify, of course, only the 
general laws of motion — not special laws, as gravitation or the like. I am 
particular in dwelling on this point, because it appears to me to be too often 
forgotten. Physicists speak, for instance, sometimes of the possibility that 
liglit-waves may be propagated to an injimie distance from the source of 
light. This is simply an impossibility. That light travels, so far as it does, 
without appreciable extinction, proves that the range of motion of the 
vibrating particles of the ictlier is ver>' small indeed, compared with the 
wave-lengths ; but this range, however small, must have a definite value 
at any given distance from the source, and the total amount of motion in 
any spherical surface round the source of light would have a definite and 
constant value. But to suppose a definite amount of oscillation in an in- 
finite number of such spheres is to suppose that an infinite effect can accrue 
from a finite cause. 
