SPECTRA OF SOME OF THE BRIGHTER STARS. 
G81 
proportion. Thus, where 5 minutes’ exposure is sufficient for a first magnitude star, 
31 minutes is required to obtain a fully-ex posed spectrum of a star of the third 
magnitude. This law, however, only applies to photographic magnitudes, and must 
be modified according to the type of spectrum or the colour of the star. 
For conveniently adjusting the exposures, tables have been constructed which show 
at a glance the position of the regulator for a star of given magnitude and declination. 
It is obvious that with an instrument of high dispersion, the number of stars it is 
possible to photograph is very limited, as the long exposures required for the fainter 
stars are impracticable, and, even if possible, the definition of the lines would be 
destroyed by atmospheric tremors. 
Hence, it is at present only possible to photograph the sj)ectra of the faint stars on 
a very small scale. With an objective of 8 inches aperture and 44 inches focal length, 
and a prism of 13° refracting angle, Professor Pickering has photographed the 
spectra of stars down to the eighth magnitude. These spectra are about one centi¬ 
metre long and a millimetre broad, and though they do not show a very great amount 
of detail, they are sufficient to reveal the type of spectrum. 
With an instrument capable of photographing faint stars, a large number of spectra 
may be taken at one exposure; but, with the instruments of larger dispersion, this 
is not generally the case, as there are few bright stars of nearly equal magnitude 
sufficiently close together. 
The red stars, being much weaker in blue and violet rays than the yellow or white 
stars, require much longer exposures than white stars of equal magnitude. To obtain 
a spectrum of /3 Pegasi extending to the K line, for example, at least three times the 
exposure required by a white star of similar magnitude must be given. 
Electrical Control. 
In consequence of the great accuracy required in the driving of the telescope when 
long exposures are necessary, the 10-inch equatorial has been fitted with a simple and 
inexpensive form of electrical control. This is a modification of that designed Ijy 
Mr. Russell, of the Sydney Observatory/^ The existing driving gear has been 
altered so that the driving rod performs its revolution in a second, and the motion is 
then communicated, to the driving screw through a small worm wlieel. The driving 
rod is vertical and in two parts, the lower portion ending in a faced ratchet wheel, 
3 inches in diameter, and with 200 teeth. The upper part of the rod ends in an arm 
at right angles to itself, and this arm carries a ratchet of suitable shape held down by 
an adjustable spring. An electro-magnet, connected with the controlling pendulum, 
is arranged so as to only permit the ratchet to pass it once a second (see fig. 3). If 
the clock be driving too quickly, the ratchet is held until the stop is raised by the 
* ‘]\roTitlily Notices,’ vol. 51, p. 4.3, 1890-91. 
MDCCCXCIII.—A 4 S 
