CELESTIAL SPECTROSCOPY. 93 
ing the importance of obtaining the spectrum of as many stars as pos- 
sible on an extended scale without an exposure inconveniently long, 
wisely determined to limit the part of the spectrum on the plate to the 
region for which the ordinary silver-bromide gelatine plates are most 
sensitive,—namely, to a small distance on each side of G,—and to em- 
ploy as the line of comparison the hydrogen line near G, and recently 
also certain lines of iron. The most minute and complete mechanical 
arrangements were provided for the purpose of securing the absolute 
rigidity of the comparison spectrum relatively to that of the star, and 
for permitting temperature adjustments and other necessary ones to be 
made. 
The perfection of these spectra is shown by the large number of lines, 
no fewer that two hundred and fifty in the case of Capella, within the 
small region of the spectrum on the plate. Already the motions of 
about fifty stars have been measured with an accuracy, in the case of 
the larger number of them, of about an English mile per second. 
At the Lick Observatory it has been shown that observations can be 
made directly by eye with an accuracy equally great. Mr. Keeler’s 
brilliant success has followed in great measure from the use of the third 
and fourth spectra of a grating 14,438 lines to the inch. The marvel- 
lous accuracy attainable in his hands on a suitable star is shown by 
observations on three nights of the star Arcturus, the largest diverg- 
ence of his measures being not greater than six-tenths of a mile per 
second, while the mean of the three nights’ work agreed with the mean 
of five photographic determinations of the same star at Potsdam to 
within one-tenth of an English mile. These are determinations of the 
motions of a sun so stupendously remote that even the method of 
parallax practically fails to fathom the depth of intervening space, and 
by means of light-waves which have been, according to Elkin’s nominal 
parallax, nearly two hundred years upon their journey. 
Mr. Keeler, with his magnificent means, has accomplished a task 
which J attempted in vain in 1874, with the comparatively poor appli- 
ances at my disposal, of measuring the motions in the line of sight of some 
of the planetary nebule. As the stars have considerable motions in 
space, it was to be expected that nebulie should possess similar motions, 
for the stellar motions must have belonged to the nebul out of which 
they have been evolved. My instrumental means, limiting my power 
of detection to motions greater than 25 miles per second, were insufti- 
cient. Mr. Keeler has found in the examination of ten nebule motions 
varying from 2 miles to 27 miles, with one exceptional motion of nearly 
40 miles. 
For the nebula of Orion, Mr. Keeler finds a motion of recession of 
about 10 miles a second. Now, this motion agrees closely with what it 
should appear to have from the drift of the solar system itself, so far 
as it has been possible at present to ascertain the probable velocity of 
the sun in space, This grand nebula, of vast extent and of extreme 
