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PROFESSOR J, N. LOCKTER ON THE PHOTOGRAPHIC 
II. METHODS or WORK. 
(1.) The Objective Prisms. 
At Kensington arrangements have been made for photographing the spectra by 
means of objective prisms. This method was originally suggested and employed by 
Fraunhofer, in 1814, for observing stellar spectra. For photographic purposes the 
method, which is obviously the best, has been revived with conspicuous success by 
Professor Pickering, of Harvard College. 
An ordinary telescope with a prism in front of the object-glass becomes a complete 
star spectroscope, the rays coming from a star being already parallel, so that both slit 
and collimating lens can be dispensed with. As the image of a star is a point, the 
spectrum thus obtained will have no breadth, and for eye observations it is necessaiy 
to use a cylindrical lens in conjunction wuth the eye-piece of the telescope. For 
photographic purposes, however, a different method is adopted. The prism is so 
placed that its refracting edge lies in a parallel of declination, and then it is only 
necessary to allow the driving clock to be slightly in error in order to obtain the 
necessary width. 
Instrument A .—The main instrument employed in the work at Kensington has 
been a 6-inch refracting telescope, with an object-glass made and corrected for G by 
the Brothers Henry. This was at first used in conjunction with a prism of by 
Hilger, and, for convenience, this combination will be afterwards referred to as 
Instrument A, The object-glass and prism are fixed at the end of a wooden tube, 
which is attached to the side of the 10-inch equatorial, at such an angle that the 
spectrum of a star falls on the middle of the photographic plate when its image is at 
the centre of the field of the larger instrument. The camera is arranged to take 
plates of the ordinary commercial size, 4^ X 3j: inches. The spectra obtained with 
this instrument are 0’6 inch long from F to K, An excellent photograph of the 
spectrum of a first magnitude star can be obtained with an exposure of 5 minutes. 
Instrument B. —A 6-inch prism, with a refracting angle of 45°, was obtained from 
the Brothers Henry in May last, and, since then, this has been in constant use with 
the Henry 6-inch object-glass. The combination will be referred to as Instrument B. 
The spectra obtained with this instrument are 2 inches long from F to K, and the 
definition is exquisite. In some photographs the calcium line at H is very clearly 
separated from the line of hydrogen, which occupies very nearly the same position. 
It is unnecessary to swing the back of the camera in order to get a perfect focus from 
F to K, The deviation of the prism is so great that it would be very inconvenient to 
incline the tube which supports it at the proper angle to the larger telescoj^e. When 
photographing the spectrum of a star, the star is therefore first brought to the centre 
of the field of the large telescope, and the proper deviation is then given by reading 
off on the declination circle. This method has been found to work quite satisfactorily. 
