SPECTRA OF SOME OF THE BRIGHTER STARS. 
679 
With this combination the exposure required for a first magnitude star is about 
20 minutes. The method of mounting the prism is shown in fig. ]. 
Fig. 1. 
Objective prism of 4.“)° fitted to 6-incb object-glass. 
Instrument C .—Two prisms of 74° each and 10 inches clear aperture have been 
provided for use with the 10-inch Cooke refractor, but, as this telesco23e is required for 
the instruction of students during the greater part of the year, they have not been 
largely employed. The prisms are circular in form, and are fitted in cells provided 
with adjusting screws. A camera taking plates 4^ X 3^ inches replaces the eye¬ 
piece of the telescope. 
To facilitate the adjustment of the spectrum, the 3-inch finder can be inclined to the 
body of the telescope, so that when a star is brought on the cross-wires its spectrum 
falls in the proper place on the photographic plate. The focal length of the 10-inch 
objective is about 11 feet, and the spectra obtained are about 1^ inches long from F to 
K. With this dispersion and aperture, a normal first magnitude star, like Altair, 
requires an exposure of about 10 minutes on a fairly good night to give a fully 
exposed spectrunv iV^li of ao inch wide. The results so far obtained with this 
instrument do not equal those obtained 'with Instrument B. Fig. 2 shows the way in 
which the prisms are attached to the cell containing the object-glass. 
Instrument D .—For the fainter stars, the 6-inch prism of 7-g-°, which was dis¬ 
mounted from Instrument A, has been adapted to a Dallmeyer rectilinear lens of 
6 inches aperture and 48 inches focal length, the tube with its appendages having 
been mounted in place of the tube of the Dallmeyer photoheliograph. This, also, is 
