416 
ME. W. HUGGINS AND DE. W. A. MILLEE ON 
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 f 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 collimating lens g , made by T. Boss ; 
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, A, each having a 
refracting angle of about 60° ; they were made by T. Boss, and are of very dense and 
homogeneous flint glass. The prisms are supported upon a suitable mounting, which 
permits them to be duly levelled and adjusted. Since the feebleness 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 minimum devia- 
tion 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 .1, furnished with an 
object-glass of 0-8 inch diameter and 6 "75 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, crossing 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slipped easily over the eyepiece of the telescope, and was used or withdrawn as was 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 approximatively at the point of intersec- 
tion of the red and the violet pencils from the last prism ; consequently it falls within 
drical surface should he parallel with the direction of the slit. The line of light is in this case formed by the 
lens ; and the length of this line, corresponding to the visible breadth of the spectrum, is equal to the diameter 
of the cone of rays from the object-glass where they fall upon the slit. With this arrangement, the spectrum 
appears to he spread out, in place of being contracted at the two extremities. Owing to the large amount of 
dispersion to which the light is subjected, it was judged unadvisable to weaken still further the already feeble 
illumination of the extremities of the spectrum ; and in the examination of the 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 advan- 
tage which this possesses over the convex form is more than balanced by the inconvenience of the increased 
length given to the whole apparatus. 
