478 Royal Astronomical Society, 



are the best judge ; and I shall therefore merely give you the means 

 of examining the pretensions of the instrument. In this view I have 

 made three sketches of the model with the camera lucida, and I have 

 added to each the same letters of reference to the same parts. I 

 transmit to you also the opinion of Carlini of Milan, and of his col- 

 leagues, as to this instrument, which was submitted to their ex- 

 amination some time ago. 



" I may begin by stating that the chief object of the instrument 

 is to determine the difference of right ascension between any two 

 stars, without the agency of time as an element, the equatoreal an- 

 gular difference between them being measured directly, in arc, on 

 an hour-circle, graduated in degrees and minutes for that purpose. 

 It is true that time does enter as an element into the principle of the 

 instrument, inasmuch as a certain part of the machinery is moved 

 by clock-work, in the manner used in many equatoreals ; but this 

 agency is purely mechanical and subsidiary, and does not require 

 that the absolute time should either be exactly known, or its march 

 be exactly kept. 



" The instrument is essentially an equatoreal arc, in its structure, — 

 that is to say, its principal axis is directed to the pole, and it carries 

 a telescope capable of being directed to any star which is above the 

 horizon. [I should mention, in passing, that the clock-work ma- 

 chinery is not included in the model ; and there may be observed 

 some other mechanical omissions, it not having been thought worth 

 while to encumber either the model or the description with more 

 details than are necessary to an explanation of the principle and 

 workings of the instrument.] 



" The principal or polar axis of the instrument is made hollow — 

 in fact, is a telescope, having at its upper extremity a small reflector 

 or speculum capable of being directed at any angle into the tube of 

 this axis telescope. The object-glass of this telescope is fixed not 

 at its extremity, but about half-way between the upper end and the 

 centre. At the centre there is placed another reflector, which stands 

 at an angle of 45° with the length of the tube, to receive the image 

 of a star formed by the object-glass from the rays reflected from the 

 upper speculum. The side of the axis telescope is perforated, in 

 order to allow the image of the star which is reflected from the central 

 speculum to pass into the middle of another telescope, which, for di- 

 stinction, may be called the declination telescope, as it is attached to, 

 and carries with it, a declination circle. In the middle of this de- 

 clination telescope there is fitted a very small reflector, at an angle 

 of 45° to its length, on which the image of the star reflected from 

 the central speculum is received and transmitted to the eye of the 

 observer, in every position of the declination telescope. 



" The further arrangements of the instrument will perhaps be 

 more readily understood by describing the manner of using it, than 

 by giving a detailed explanation of the parts. 



" In commencing an observation, the upper speculum is directed 

 to a standard star of the first or second magnitude, partly by moving 

 it on its own axis of rotation, so as to direct the rays into the prin- 



