j CELESTIAL SPECTROSCOPY. 91 
It may well be that in the very early stages condensing masses are 
subject to very different conditions, and that condensation may not al- 
- ways begin at one or two centers, but sometimes set in at a large num- 
ber of points, and proceed in the different cases along very different 
lines of evolution. 
Invisible Motions revealed by the Spectroscope.—Besides its more direct 
use in the chemical analysis of the heavenly bodies, the spectroscope 
has given to us a great and unexpected power of advance along the 
lines of the older astronomy. In the future a higher value may indeed 
be placed upon this indirect use of the spectroscope than upon its 
chemical revelations. 
By no direct astronomical methods could motions of approach or of 
recession of the stars be even detected, much less could they be meas- 
ured. A body coming directly toward us or going directly from us ap- 
pears to stand still. In the case of the stars we can receive no assist- 
ance from change of size or of brightness. The stars show no true 
disks in our instruments, and the nearest of them is so far off that if it 
were approaching us at the rate of a hundred miles in a second of time, 
a whole century of such rapid approach would not do more than increase 
its brightness by the one-fortieth part. 
Still it was only too clear that so long as we were unable to ascer- 
tain directly those components of the stars’ motions which lie in the 
line of sight, the speed and direction of the solar motion in space, and 
many of the great problems of the constitution of the heavens, must 
remain more or less imperfectly known. Now the spectroscope has 
placed in our hands this power, which, though so essential, appeared 
almost in the nature of things to lie forever beyond our grasp; it 
enables us to measure directly, and under favorable circumstances to 
within a mile per second, or even less, the speed of approach or of re- 
cession of a heavenly body. This method of observation has the great 
advantage for the astronomer of being independent of the distance of 
the moving body, and is therefore as applicable and as certain in the 
case of a body on the extreme confines of the visible universe (so long 
as it is bright enough), as in the case of a neighboring planet. 
Doppler had suggested as far back as 1841, that the same principle 
on which he had shown that a sound should become sharper or flatter 
if there were an approach or a recession between the ear and the source 
of the sound, would apply equally to light; and he went on to say that 
the difference of color of some of the binary stars might be produced 
in this way by their motions. Doppler was right in that the principle 
is true in the case of light, but he was wrong in the particular con- 
clusion which he drew from it. Even if we suppose a star to be mov- 
ing with a sufficiently enormous velocity to alter sensibly its color to 
the eye, no such change would actually be seen, for the reason that 
the store of invisible light beyond both limits of the visible spectruim, 
