530 
ME. W. HUGGINS ON THE SPECTEA 
It appeared premature at the time to refer to these negative results, as it did not 
seem to be probable that the stars were moving with velocities sufficiently great to cause 
a change of refrangibility which could be detected with our instrument. The insuffi- 
ciency of our apparatus for this very delicate investigation does not, however, diminish 
the trustworthiness of the results we obtained respecting the chemical constitution of 
the stars, as the evidence for the existence or otherwise of a terrestrial substance was 
made to rest upon the coincidence, or want of coincidence in general character as well 
as position of several lines , and not upon that of a single line. 
According to the undulatory theory, light is propagated with equal velocity in all 
directions, whether the luminous body be at rest or in motion. The change of refran- 
gibility is therefore to be looked for from the diminished or increased distance the 
light would have to traverse if the luminous object and the observer had a rapid motion 
towards or from each other. The great relative velocity of light to the known planetary 
velocities, and to the probable motions of the few stars of which the parallax is known, 
showed that any alterations of position which might be expected from this cause in the 
lines of the stellar spectra would not exceed a fraction of the interval between the 
double line D, for that part of the spectrum. 
I have devoted much time to the construction and trial of various forms of apparatus 
with which I hoped to accomplish the detection of so small an amount of change of 
refrangibility. The difficulties of this investigation I have found to be very great, and 
it is only after some years that I have succeeded in obtaining a few results which I hope 
will be acceptable to the Royal Society. 
The subject of the influence of the motions of the heavenly bodies on the index of 
refraction of light had already, at the time of the publication of our paper in 1864, 
occupied the attention of Mr. J. C. Maxwell, F.R.S., who had made some experiments 
in an analogous direction. In the spring of last year, at my request, Mr. Maxwell 
sent to me a statement of his views and of the experiments which he had made. I have 
his permission to enrich this communication with the clear statement of the subject 
which is contained in his letter, dated June 10, 1867. 
In 1841, Doppler showed that since the impression which is received by the eye or 
the ear does not depend upon the intrinsic strength and period of the waves of light and 
of sound, but is determined by the interval of time in which they fall upon the organ 
of the observer, it follows that the colour and intensity of an impression of light, and 
the pitch and strength of a sound will be altered by a motion of the source of the light 
or of the sound, or by a motion of the observer, towards or from each other *. 
Doppler endeavoured by this consideration to account for the remarkable differences 
of colour which some of the binary stars present, and for some other phenomena of the 
heavenly bodies. That Doppler was not correct in making this application of his theory 
is obvious from the consideration that even if a star could be conceived to be moving with 
* “ TJeber das farbige Iicbt der Doppelsterne und einiger anderer Gestime des Himmels,” Bobm. Gesell. Abh. ii. 
1841-42, s. 465. 
