Spectra of some of the Fixed Stars. 409 



In front of the slit d, and over one half of it, is placed a right- 

 angled prism e, for the purpose of reflecting the light which it 

 receives from the mirror /through the slit. In the brass tube 

 c are two holes : by one of these the light is allowed to pass from 

 the mirror to the reflecting-prism e ; and by means of the other, 

 access to the milled head for regulating the width of the slit is 

 permitted. Behind the slit, and at a distance equal to its focal 

 length, is placed an achromatic collim'ating lens g, made by 

 T. Ross ; this has a diameter of 0*6 inch, and a focal length of 

 4^ inches. These proportions are such that the lens receives 

 the whole of the light which diverges from the linear image of 

 the star when this is brought exactly within the jaws of the slit. 



The dispersing portion of the apparatus consists of two prisms, 

 h, each having a refracting angle of about 60° ; they were made 

 by T. Ross, and are of very dense and homogeneous flint glass. 

 The prisms are supported upon a suitable mounting, which per- 

 mits them to be duly levelled and adjusted. Since the feeble- 

 ness of the light from the stars limits the observations for 

 the most part to the central and more luminous portions of the 

 spectrum, the prisms have been adjusted to the angle of mini- 

 mum deviation for the ray D. A cover of brass, k, encloses 

 this part of the apparatus; and by this means the prisms are 

 protected from accidental displacement and from dust. 



The spectrum is viewed through a small achromatic telescope 

 /, furnished with an object-glass of 08 inch diameter and 675 

 inches focal length. This telescope has an adjustment for level 

 at m. The axis of the telescope can be lowered and raised, and 

 the tube can be also rotated around the vertical axis of support 

 at n. At the focus of the object-glass are fixed two wires, cross- 

 ing at an angle of 90°. These are viewed, together with the 

 spectrum, by a positive eyepiece p, giving a magnifying power 

 of 5*7 diameters. As the eyes of the two observers do not 

 possess the same focal distance, a spectacle-lens, corresponding 

 to the focal difference between the two, was fitted into a brass 

 tube, which slipped easily over the eyepiece of the telescope, 

 and was used or withdrawn as was necessary. 



This telescope, when properly adjusted and clamped, is carried 

 by a micrometer-screw q, which was constructed and fitted to the 

 instrument by Cooke and Sons. The centre of motion about 

 which it is carried is placed approximative^ at the point of in- 

 stellar spectra the position of the cylindrical lens with its axis at right 

 angles to the slit, as mentioned in the text, was therefore adopted. 



A plano-concave cylindrical lens of about 14 inches negative focal length 

 was also tried. The slight advantage which this possesses over the convex 

 form is more than balanced by the inconvenience of the increased length 

 given to the whole apparatus. 



