and a 15 inch mirror, which is one of the most perfect reflectors
constructed by Dr. Draper, and with which his photograph of the moon was
taken. The first two instruments mentioned above have been kept at work
during the first part of every clear night for several months. It is now
intended that at least three telescopes shall be used during the whole
night, until the work is interrupted by daylight.

The spectra have been produced by placing in front of the telescope a
large prism, thus returning to the method originally employed by
Fraunhofer in the first study of stellar spectra. Four 15° prisms have
been constructed, the three largest having clear apertures of nearly
eleven inches, and the fourth being somewhat smaller. The entire weight
of these prisms exceeds a hundred pounds, and they fill a brass cubical
box a foot on each side. The spectrum of a star formed by this apparatus
is extremely narrow when the telescope is driven by clockwork in the
usual way. A motion is accordingly given to the telescope slightly
differing from that of the earth by means of a secondary clock
controlling it electrically. The spectrum is thus spread into a band,
having a width proportional to the time of exposure and to the rate of
the controlling clock.

This band is generally not uniformly dense. It exhibits lines
perpendicular to the refracting edge of the prism, such as are produced
in the field of an ordinary spectroscope by particles of dust upon the
slit. In the present case, these lines may be due to variations in the
transparency of the air during the time of exposure, or to instrumental
causes, such as irregular running of the driving clock, or slight
changes in the motion of the telescope, resulting from the manner in
which its polar axis is supported.