﻿186 DESCRIPTION OF THE OBSERVATORY AT CAMBRIDGE. 



There are, however, one or two points in which the instrument is susceptible of im- 

 provement. The arrangement of the divisions of both the declination and hour-circles 

 is awkward, and the reading off of both attended with needless trouble. The screw for 

 adjusting the focus of the eye-pieces has too fine a thread and is inconveniently situated. 

 The clock, for regulating the movement of the telescope is disproportionate to the other 

 parts of the instrument, and in cold weather requires frequent adjustment. 



In every other particular, I believe we have reason to be perfectly satisfied. The 

 object-glass, by far the most important part of the instrument, is an admirable specimen 

 of the skill of Messrs. Merz & Son. Its defining excellence is such, that powers of from 

 seven to twelve hundred are habitually used in the measurement of double stars. On 

 rare occasions, a power of two thousand has shown well the disks of Neptune and of the 

 satellites of Jupiter. With powers of seven and eight hundred, we have separated the 

 disks of stars whose measured distance has proved to be 0."3. 



With a power of one hundred and forty-one, the sixth star of the trapezium of Orion 

 is distinctly separated from its bright companion. The fifth star we have seen after 

 sunrise. This star is classed as of the thirteenth magnitude. 



The satellites of Neptune and the inner and eighth satellites of Saturn are seen 

 steadily, as well as the edge of its ring, at what is called its disappearance. 



I consider the smallness of the disks of the stars which are given by this object-glass 

 an indication of remarkable excellence. Soon after mounting the telescope, we had sev- 

 eral caps made for reducing the aperture. These we have since entirely discarded, hav- 

 ing found on trial that the whole aperture may be used without injury to the definition. 



The Transit Circle. — The meridian-circle in some respects resembles the circle de- 

 signed and made by Troughton for the Imperial Academy at St. Petersburg, which 

 ultimately, in the hands of Mr. Groombridge at Blackheath, rendered such important 

 services to astronomy. Many improvements have, however, been introduced. To the 

 Groombridge circle there were four reading microscopes, which were supported by long 

 brass arms. The Cambridge circle has eight micrometer reading microscopes, and these 

 are attached immediately to the granite pier. For the plumb-line, a striding level is 

 substituted, which, combined with the method of reflection from quicksilver at the nadir 

 point, affords an independent means of ascertaining the amount of collimation of the 

 mid wire without reversal of the pivots ; there is, however, apparatus for reversing the 

 instrument. 



Beside the usual mode of illuminating the field through the axis, there are facilities 

 for illuminating the wires in a dark field. The eye-piece is provided with two microme- 

 ters, one having a vertical and the other a horizontal movement; double friction -wheels, 



