666 
Observatory are used on practically every 
clear night, the amount of work accom- 
plished is very considerable, notwithstand- 
ing the smallness of the observing force, 
while the variety of instruments makes the 
field covered quite a wide one. 
THE THIRTY-SIX-INCH REFRACTOR. 
This splendid instrument is in perfect 
order. A small periodic error originally in 
the driving-gear has been almost wholly 
corrected by Professor Campbell (who has 
charge of the instrument), by introducing 
into the clock- train a periodic error of equal 
amount but opposite sign, so that the two 
errors neutralize each other. 
About half the time of the 36-inch equa- 
torial is devoted to spectroscopic determina- 
tions of the motions of stars in the line of 
sight, with the aid of the Mills Spectrograph. 
This department of the scientific work of 
the Observatory, which is probably as im- 
portant as any work that can be done at 
the present time with a large telescope, has 
been admirably systematized by Professor 
Campbell, who is assisted in the observa- 
tions and reductions by Mr. Wright. The 
probable error of a single determination is 
only about 0.25 kilometer for the best stars, 
a degree of accuracy which has never be- 
fore been reached in such measurements. 
A correcting lens, which is placed in the 
cone of rays from the 36-inch objective, and 
which changes the chromatic aberration of 
the telescope so as to adapt it to photo- 
graphic work, has added somewhat to the 
accuracy and very much to the convenience 
of the observations. 
During the past year 522 spectrograms 
were obtained. Over 200 spectrograms 
were definitively, and about 500 approxi- 
mately, measured and reduced. 
In connection with this work, the follow- 
ing stars have been found to have variable 
velocities in the line of sight, and therefore 
to be revolving stellar systems : 
SCIENCE. 
[N. 8. Vou. X. No. 254. 
n Pegasi, y Draconis, 0 Leonia, & Gemi- 
norum, « Pegasi, @ Draconis, « Libre, £ 
Capricorni, h Draconis, 2 Andromede, « 
Ursz Minoris, » Draconis, « Urs Minoris, 
a Aurigee, v Sagittarii. 
From 25 to 30 spectrograms of each of 
these stars are needed for determining the 
orbits. The observations of several of them 
are essentially complete, and the others are 
being observed as rapidly as circumstances 
will permit. An investigation of the orbit 
of 7 Aquile, based on all the available data, — 
has been made by Mr. Wright, who has 
also partially computed a set of tables for 
facilitating the reduction of spectroscopic 
observations. 
Two of the stars in the above list are of 
special interest. In thecase of Polaris the 
velocity in the line of sight has a double 
period; a short oscillation, having a range 
of four miles per second and a complete 
period of 3 days 23 hours, is superposed on 
a much longer one, the period of which has 
not yet been determined, though it is prob- 
ably as much as several years. The star 
is therefore a part of a triple system, two of 
the bodies being dark. Capella, or « Auri- 
gee, has a double spectrum. The two sets 
of lines undergo a periodic relative displace- 
ment. 
To aid in the observations and reductions 
in this important part of the Observatory’s 
work another assistant is greatly needed. 
It is a severe strain on both the present ob- 
servers to keep the work in as satisfactory 
a state as it is at present. 
A special study by Professor Campbell of 
the triple hydrogen lines in the spectrum 
of o Ceti led to many interesting results. 
The 36-inch telescope has also been used 
for other spectroscopic investigations. The 
spectra of all sufficiently bright comets have 
been photographed by Mr. Wright. Obser- 
vations of nebular spectra have been made 
by the Director and others. A comparison 
of the spectrum of hydrogen in a Geissler 
