92 CELESTIAL SPECTROSCOPY. 
the blue and the red, would be drawn upon, and light-waves invisible 
to us would be exalted or degraded so as to take the place of those 
raised or lowered in the visible region, and the color of the star would 
remain unchanged. About eight years later, Fizeau pointed out the 
importance of considering the individual wave-lengths of which white 
light is composed. As soon however as we had learned to recognize 
the lines of known substances in the spectra of the heavenly bodies, 
Doppler’s principle became applicable as the basis of a new and most 
fruitful method of investigation. The measurement of the small shift 
of the celestial lines from their true positions, as shown by the same 
lines in the spectrum of a terrestrial substance, gives to us the means 
of ascertaining directly in miles per second the speed of approach or 
of recession of the heavenly body from which the light has come. 
An account of the first application of this method of research to the 
stars, which was made in my observatory in 1868, was given by Sir 
Gabriel Stokes from this chair at the meeting at Exeter in 1869. The 
stellar motions determined by me were shortly after-confirmed by Prof. 
Vogel in the case of Sirius, and in the case of other stars by Mr. 
Christie, now astronomer-royal, at Greenwich; but, necessarily, in 
consequence of the inadequacy of the instruments then in use for so 
delicate an inquiry, the amounts of these motions were but approximate. 
The method was shortly afterwards taken up systematically at Green- 
wich and at the Rugby Observatory. It 1s to be greatly regretted that, 
for some reasons, the results have not been sufficiently accordant and 
accurate for a research of such exceptional delicacy. On this account 
probably, as well as that the spectroscope at that early time had 
scarcely become a familiar instrument in the observatory, astronomers 
were slow in availing themselves of this new and remarkable power of 
investigation. That this comparative neglect of so truly wonderful a 
method of ascertaining what was otherwise outside our powers of ob- 
servation has greatly retarded the progress of astronomy during the 
last fifteen years, is but too clearly shown by the brilliant results which 
within the last couple of years have followed fast upon the recent mas- 
terly application of this method by photography at Potsdam, and by 
eye with the needful accuracy at the Lick Observatory. At last this 
use of the spectroscope has taken its true place as one of the most 
potent methods of astronomical research. It gives us the motions of 
approach and of recession, not in angular measures, which depend for 
their translation into actual velocities upon separate determinations of 
parallactic displacements, but at once in terrestrial units of distance. 
This method of work will doubtless be very prominent in the astron- 
omy of the near future, and to it probably we shall have to look for the 
more important discoveries in sideral astronomy which will be made 
during the coming century. 
In his recent application of photography to this method of determin- 
ing celestial motions, Prof. Vogel, assisted by Dr. Scheiner, consider- 
