OF SOME OF THE STARS AND NEETTL7E. 
5B3 
As an example, let ns suppose the star to be fixed and the earth to be moving directly 
away from the star with the velocity due to its motion round the sun. The coefficient 
of aberration indicates that the velocity of light is about 10,000 times that of the earth 
in its orbit, and it appears from the observations of Angstrom and Ditscheiner that the 
wave-length of the less refrangible of the lines forming D exceeds that of the other by 
about one-thousandth part of itself. Hence, if the lines corresponding to D in the light 
of the star are due to sodium in the star, these lines in the starlight will be less refran- 
gible than the corresponding lines in a terrestrial sodium-flame by about a tenth part 
of the difference between D, and D 2 . 
When the earth is moving towards the star, the lines will be more refrangible than 
the corresponding terrestrial lines by about the same quantity. 
The effect of the proper motion of stars would of course have to be compounded with 
the effect of the earth’s own motion, in order to determine the velocity of approach or 
separation. 
To observe these differences of the light from stars, a spectroscope is necessary, that is, 
an instrument for separating the rays of different periods ; and it is immaterial in what 
direction the refraction of the light through the prisms takes place, because the period 
of the light is the thing to be observed by comparison with that of a terrestrial flame. 
If, instead of a spectroscope, an achromatic prism were used, which produces an equal 
deviation on rays of different periods, no difference between the light of different stars 
could be detected, as the only difference which could exist is that of their period. 
If the motion of a luminiferous medium in the place where the experiment is made is 
different from that of the earth, a difference in the deviation might be expected according 
to the direction of the ray within the prisms, and this difference would be nearly the 
same whatever the source of the light. 
There are therefore two different and independent subjects of experiment. The one 
is the alteration in the period of vibration of light due to the relative motion of the stars 
and the earth. The fact of such an alteration is independent of the form under which 
we accept the theory of undulations, and the possibility of establishing its existence de- 
pends on the discovery of lines in the stellar spectra, indicating by their arrangement that 
their origin is due to the existence of substances in the star having the same properties 
as substances found on the earth. Any method of observing small differences in the 
period of vibration of rays, if sufficiently exact, will enable us to verify the theory, and 
to determine the actual rate of approach or separation between the earth and any star. 
The other subject of experiment is the relation between the index of refraction of a 
ray and the direction in which it traverses the prism. The essentials of this experi- 
ment are entirely terrestrial, and independent of the source of light, and depend only on 
the relative motion of the prism and the luminiferous medium, and on the direction in 
which the ray passes through the prism. 
The theory of this experiment, however, depends on the form in which we accept the 
theory of undulations. In every form of the theory, the index of refraction depends on 
