64 Grupp—On a New Form of Equatorial Telescope. 
of the three components. A variation in distance between the large crown and 
the small compound corrector gives the correction for achromatism, and a varia- 
tion in distance between the two components of the corrector gives a correction for 
spherical aberration. 
It can be easily understood that this arrangement possesses special charms for 
amateurs whose stock of tools is limited. 
The form of instrument I would propose for a large size is shown in the accom- 
panying plate (Plate 2), which is drawn to represent one of about 27-inch aperture. 
The mechanical part consists simply of a polar axis working im a polar pillar P, 
through which at its upper end passes an axis on which swing a pair of light-framed 
girders, supporting at one end the light crown lens of the objective and at the other a 
counterpoise. These girders we may call the tube. At the mtersection of the axes is placed 
the mirror, mounted on the same axes. By a simple mechanical arrangement. this 
mirror is carried round simultaneously with the tube, but at half its angular velo- 
city, ¢.e., for every 10° the tube (or girders) carrying the objective swings, the 
mirror rotates 5°. The result is that the rays are always reflected in a direction 
towards the pole, ¢.e., in a continuation of the polar axis; and this evidently is 
true no matter what the position of the telescope may be either in A.R. or declina- 
tion. In the plate the rays are represented as passing through the walls of the 
observatory, where the eye-piece is arranged in an eminently comfortable position. 
The simultaneous motion of tube and mirror is effected, not by gearing, but by a 
system of steel bands, which work very smoothly and without loss. The correct- 
ing lens is placed at, or near, the cube. 
For the motions of the instrument, steel bands pass round large pulleys, both on A. R. 
and declination axes; these are carried into the lower apartments of the observa- 
tory, and attached to the pistons of hydraulic cylinders, the taps of which are con- 
nected by long rods with handles in the observer’s room, so that he has from his seat 
complete control over the instrument, without any more manual labour than is 
necessary to open a water tap. 
For the setting of the instrument. There are no circles, but the steel bands which 
drive the instrument are connected to wires which pass to the upper storey of 
the building over pulleys, and are connected to indices which rise and fall on two 
scales attached to the wall, and these indicate to the observer the position of the 
instrument. It may be objected that this method would be open to inaccuracy 
from variation in temperature of the bands, &c.; but by setting once or twice each 
evening on some known object any zero error could be sufficiently eliminated.* 
Clockwork is shown, applied to drive the instrument; but I am inclined to hope 
that even this would be unnecessary, and that the water could be made to drive 
the instrument by a simple form of Clypsedra. 
.* Since the above Paper was read, Dr. Huggins, '.R.S., has proposed to the author to have these 
seales painted with luminous paint, so as to be visible to the observer without the necessity of illumina- 
tion. 
