252 PROFESSOR L. BECKER ON 
ago, is remarkable in so far as its inclined stand anticipates the chief structural 
feature of the Potsdam astrographic refractor. The reflector, which had proved useless 
in its old condition, obtained in 1895 a new silver-on-glass mirror, driving clock with — 
electric control, driving sector and declination clamp with slow motion, all from the 
works of Sir Howard on The mounting of the spectrograph was supplied by a 
local smith. A platform forming a right-angled triangle extends from the upper end of — 
the tube to the free end of the declination axis, and its plane is inclined 35 degrees to” 
the optical axis. Parallel to it, the central ray of the reflector is reflected by a plane 
mirror. The platform is a stiff structure for its weight. It consists of two layers of 
corrugated iron, with the corrugations crossed and bolted at every point of contact, and 
it is strengthened by thin sheet steel ribs. To it is clamped a quarter-inch steel sole- 
plate, with adjustable bearings for the two tubes of the spectrograph, and on this 
sole-plate a small cast-iron table carrying the prism-box can be adjusted and clamped. 
The platform rests at its upper end, a corner of the triangle, on a casting which is bolted 
to the tube of the reflector; at its lower end, the shortest side of the triangle, it is 
screwed to a strong cast-iron arm, which is fixed to the declination axis, in place of the 
balancing weights, at right angles to this axis and the axis of the tube. As I had the 
declination axis lengthened, and the telescope tube shortened and placed more favourably 
in its cradle, the movable part of the instrument weighs now less than in its old condition. — 
The object-glass of the collimator has an aperture of 8°2 em. and focal length of 
74 cm. ; that of the camera, a Cooke triplet, 8°9 cm. by 149 cm. The focal length of 
both combined has a large temperature coefficient, 0°13 mm. for a degree centigrade. 
The prism made by Hilger of white Jena flint glass measures 16°5 cm. on a side, and is 
9°5 cm. in height. Since it was re-annealed its separating power is most satisfactory. 
The central portion of the spectrograph is enclosed in a box, and by means of a small 
heating apparatus the temperature of the prism and the object-glasses can, at least to— 
some extent, be kept under control. Unfortunately, the instrument cannot be used in 
summer after a sunny day, because in the iron dome the large prism is heated in such a 
way that the definition becomes too bad for accurate work. 
The jaws of the sht are formed by the two halves of a circular mirror 2°5 em. in 
diameter, and they open symmetrically 0°15 mm. for a revolution of the screw. The 
width here employed was usually 0°018 mm. ‘The plane of the mirrors which form the 
jaws of the slit is inclined 7 degrees to the plane normal to the central ray. If the 
image of the star does not fall on the slit, the rays are reflected towards a small mirror 
which is fixed to the telescope tube, and thence towards a viewing “telescope” (which 
is focussed on the slit) of 7 cm. aperture and 30 cm. focal length (two object-glasses— 
mounted close together). It lies almost parallel to the collimator. Owing to the large 
size of the jaws of the slit, the effective field is half a degree, which is a great con- 
venience in finding a star and setting it on the slit. The spark apparatus is hinged to 
the platform in front of the slit. When turned into position, the optical axis of its lens 
coincides with that of the collimator. . 
