SPECTRUM ANALYSIS OF THE HEAVENLY BODIES. 155 
or by that of the earth, so that the light which emanates from the source ¢ 
seconds afterwards reaches the earth in a time 'T’. 
“During the ¢ seconds n ¢ vibrations of the source of light took place, and 
these reached the earth between the time T and the time t+", that is, 
during ¢+T’—T seconds. The number of vibrations which reached the 
t 
“Tf v is the velocity of separation of the source of light from the earth, and 
V the velocity of the light between the bodies, then v t= V(T’—T), and the 
earth per second was, therefore, no longer », but n 
number of vibrations per second at the earth will be n vig? 
“Tf, therefore, the light of a star be due to sodium, or be absorbed by vapour 
of sodium, then the light, when it reaches the earth, will have an excess or 
defect of rays whose period of vibration is to that of the period of sodium as 
V-+-v is to Y.” 
There is another quite distinct way in which, under some circumstances, 
it is conceivable that an alteration of refrangibility might take place. If 
the motion of the luminiferous medium be different from that of the earth, 
that is, does not accompany the earth in its motion, a deviation might be 
expected, according to the direction of the ray within the prisms. The essen- 
tials of this experiment are entirely terrestrial, and depend only on the 
relative motion of the prism and the luminiferous medium, and on the direc- 
tion in which the ray passes through the prism. Mr. Maxwell has made 
some careful experiments in this direction, but has obtained only negative 
results. 
Several of my early attempts failed in consequence of insufficient disper- 
Sive power in the apparatus employed. It was only after several quite dis- 
tinct forms of apparatus were successively constructed, that I succeeded in 
obtaining an instrument suitable for these very delicate observations. 
A serious difficulty presented itself from the inconvenience, and in some 
degree untrustworthiness for this investigation, of the ordinary method of 
reflecting into the instrument a spectrum of comparison, by means of a small 
prism placed over one half of the slit, even when the precautions described 
in my papers presented to the Royal Society, for the purpose of ensuring 
perfect accuracy in relative position in the instrument of the star-spectrum 
with that to be compared with it, were carefully observed. 
A description of the apparatus which I finally adopted, and which appears 
in all respects trustworthy, is contained in the paper presented to the Royal 
Society. This apparatus gave an amount of dispersion equal to about seven 
prisms of dense flint glass of 60°. 
The chief difficulties I encountered arose from the unsteadiness of our 
atmosphere. Stars of the first and second magnitude give sufficient light for 
examination with the large spectroscope described; but unless the air is ex- 
ceptionally steady, the lines are seen too fitfully to permit of any certainty 
in the determination of coincidence, of the great degree of delicacy required. 
From this cause, the work of very many nights has had to be rejected. I 
will describe here those observations only which have led to a successful 
result. 
Sirius.—The brilliant light of Sirius, and the great intensity of four strong 
lines of its spectrum, made this star specially suitable for my purpose, 
though from its low altitude in our latitude, it can be successfully observed 
for an hour only on each side of the meridian. 
The coincidence of two of the strong lines of this star with those of 
