SPECTRUM ANALYSIS OF THE HEAVENLY BODIES. 153 
or of the sound, towards or from each other, is due to Doppler. 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 
wayes 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 then went wrong ; for he sought by these considerations to account 
for the remarkable difference of colour which some of the binary stars present, 
and for some other phenomena of the heavenly bodies. Now it is obvious 
that if a star could be conceived to be moving with a velocity sufficiently 
great to alter its colour sensibly to the eye, still no change of colour would 
be perceived, for the reason that, beyond the visible spectrum, at both extre- 
mities there exists a store of invisible waves which would be at the same time 
exalted or degraded into visibility to take the place of the waves which had 
been raised or lowered in refrangibility by the star’s motion. No change of 
colour, therefore, could take place until the whole of these invisible waves of 
force had been used up, which would only be the case when the relative 
motion of the source of light and_of the observer was several times greater 
than that of light. 
It is obvious from these considerations that this method of research could 
afford us information of the motion of the star only in the case in which we 
knew the period of the light at the time of its emission from the star; for then a 
comparison of this initial period with that observed at the earth would show 
the exact amount of the change of refrangibility due to the relative motions of 
the observer and the star, and as the earth’s motions are known, the motion 
of the star could be determined. 
Now this one essential condition, namely, the knowledge of the period of 
the light when emitted by the star, is fulfilled by spectrum analysis. When 
we learn the existence of a terrestrial substance in a star, we have the 
means of knowing the initial refrangibility of the dark lines in the star’s 
spectrum, which are due to the absorption of the vapour of this substance. 
It may be thought that if the lines in the spectra of the stars are subject 
to an unknown amount of displacement from the cause we have now under 
consideration, it would not be possible to make use of these lines to learn the 
star’s chemical constitution. This objection, however, does not obtain ; for 
the amount by which the lines would be displaced by any velocity we could 
with probability assign to the stars, would be too small to be even perceived 
in the spectroscopes which had hitherto been applied to the heavenly bodies. 
For example, a velocity ten times greater than that of the earth in its orbit 
would cause a line to move through a space in the spectrum about as great as 
that which separates the components of the double line D of the solar spec- 
trum. Besides this consideration, the trustworthiness of the results obtained 
by myself and Dr. Miller in our joint researches, was not allowed to rest 
upon the position of a single line, but upon the coincidence in general cha- 
racter as well as in position of a group of several lines. At the time, indeed, 
when we made our observations, we were fully aware that these direct com- 
parisons were not only of value for the determination of the chemical con- 
stitution of the stars, but that they might tell us something of the motions 
* “Ueber das farbige Licht der Doppelsterne und einiger anderer Gestirne des Him- 
mels,” Bohm. Gesell. Abh, ii. 1842-44, s, 465, 
