SPECTRA OF SOME OF THE BRIGHTER STARS. 
683 
The Slipping-plate. 
The telescope is of the Newtonian form, the eye end being provided with a slipping- 
plate, which is a modification of that designed by Dr. Common for use with his 5-foot 
reflector.* This is adapted to take a plate holder for photographing direct images, 
and also for use with eye-pieces and spectroscopes. The advantage gained by its use 
is that a small amount of motion can be given to the photographic plate or spectro¬ 
scope without in any way disturbing the large mass of the whole telescope. The 
slipping-plate is so arranged that one screw corrects any small error in right 
ascension, and the other in declination. A small eye-piece is used as a follower when 
photographing directly. It is capable of adjustment so that it may be used to observe 
any small star near the edge of the field, and being provided with cross wires illumi¬ 
nated by a small incandescent lamp it enables the observer to correct any small errors 
in right ascension or declination. 
The Slit Sj^ectroscopes. 
In addition to a Maclean star spectroscope and a small direct vision spectroscope, 
two others have been employed. One is by Hilger, and is fitted with one prism of 
60° and two half prisms. This is provided with the usual arrangements for observing 
and photographing the spectra of celestial bodies. The other is a four-prism spectro¬ 
scope by Hammersley, and is arranged so that one or two prisms may be removed at 
pleasure. It is provided with an automatic adjustment for minimum deviation, and 
is supplied with a bright line micrometer, which is illuminated electrically. A small 
telescope for. observing the spectrum by reflection from the surface of the second prism, 
a camera to replace the eye-piece, and arrangements for comparison spectra, complete 
the instrument. The whole spectroscope can be rotated round the axis of the colli¬ 
mator. In order to reduce the weight of the spectroscope the more massive parts are 
made of aluminium. The weight of the whole instrument, with the four prisms, is 
only eight pounds. 
A grating spectroscope, which has been obtained for the solar work, can also be 
adapted to the reflector for special investigations. The grating is one of Rowland’s, 
with 14,438 lines to the inch. 
(3.) Enlargements of the Negatives. 
Many of the negatives taken have been enlarged about nine times on glass, and 
further copies have been taken on bromide paper, bringing the enlargement up to 
about 25 times the size of the original. 
O 
Owing to various causes the photographic spectra obtained by the method of trails 
* ‘Monthly Notices,’ vol. 49, p. ‘297, 1888-89. 
4 s 2 
