﻿DESCRIPTION OF THE OBSERVATORY AT CAMBRIDGE. 183 



made nearly equal to the radius of the circle described by the eye-piece of the telescope. 

 The centre of curvature of the front rail is placed, relatively to the centre of motion of 

 the telescope, as the eye of the observer is to the axis k, Fig. 1 ; the upper part of the 

 rail being carried back far enough not to interfere with the motions of the telescope. 

 The centre of curvature of the back rail is placed relatively to the centre of the front 

 rail as the axis u u is to the axis k. 



The geometrical principle made use of may be illustrated by describing three circles 

 of equal radii from the apices of any triangle, a b c, as centres ; the relative positions of 

 a, b, and c being those of the observer's eye, and the axes k and u u. 



Plate IV., Fig. 6, shows the arrangement for altering the position of the part of the 

 chair supporting the head and back. Attached to the chair at x, Fig. 1, is a box for 

 holding the different eye-pieces. 



To facilitate the horizontal motion of the chair, two rails, seen in Plates II. and III., 

 of round inch-iron are let into the floor of the dome ; the inner track is on a radius of 

 seven feet four inches, and the outer of twelve feet four inches. On these rails the 

 whole frame moves on four wheels, with grooves adapted to the rail. A driving-wheel, 

 seen in Plate III., at the back part of the frame, runs on the outside track. It has a 

 double groove, one fitted to the track, and the other with spurs to prevent the slipping of 

 a rope which passes round it. This rope leads over pulleys on the top rail of the frame, 

 and passes down within reach of the observer and round a pulley fastened to the lower 

 front rail of the frame. To this pulley is attached a spiral spring, to preserve an equable 

 tension on the rope. 



This contrivance enables the observer, without leaving his seat, to move the chair 

 round on the railway, while at the same time he is able to adjust his position in altitude. 

 Whatever may be the position of the telescope, a proper movement of the driving-wheel 

 and of the endless screw brings the observer's eye to the eye-piece of the telescope. 

 The machinery above described is found on trial to fulfil perfectly the objects for 

 which it was designed, giving the observer entire command over his position, and com- 

 bining security with ease and steadiness of motion. 



The Great Refractor. — This instrument was made at the establishment of Messrs. 

 Merz & Mahler, at Munich, Bavaria. The extreme diameter of the object-glass is fif- 

 teen and a half English inches. The effective aperture is fourteen and ninety-five 

 hundredths inches, the solar focus being twenty-two feet six inches. From the outer 

 surface of the object-glass to the intersection of the declination-axis is thirteen feet 

 seven inches. From the intersection of the declination-axis to the solar focus is eight 

 feet eleven inches. 



