112 



Mr. J. P. Cooke on a Spectroscope. 



were drawn to the scale of one inch to a foot.) The parts are 

 as follows : — H is an iron tripod with levelling-screws ; G is a 



Fig. 2. 



hollow mahogany column with a conical cavity at the top ; E is 

 an iron cone which rests in the conical cavity, supporting the 

 whole body of the instrument, and connected by a long iron 

 rod with a clamping-screw beneath the tripod. By means of 

 this arrangement the instrument may be turned as a whole in 

 the horizontal plane and the collimator directed to the source 

 of light. Above the iron cone, and fastened to it securely, is 

 the main circular plate of the instrument, which is 18 inches 

 in diameter and | an inch in thickness. Near the outer edge 

 of this plate is inserted a band of silver, which is graduated 

 to 10" of arc. On the under part of the plate there is a neck, 

 and at the centre of the upper surface a socket, which are accu- 

 rately centred with each other and with the graduated limb. 

 Around the neck at E moves an iron collar, to which is attached 

 the arm bearing the observing-telescope. This moves, therefore, 

 concentric with the graduated limb, and bears a vernier, by which 

 the angular motion may be determined, reading to 10". In 

 the socket of the first plate rests the pivot of a second plate, 

 C, which turns on the first and supports the prisms with the 

 adjusting-wheel A. The diameter of the upper plate is an inch 

 less than that of the lower plate, so as to expose the graduated 

 arc near the outer edge of the latter ; and its upper surface is as 

 flat and even as possible. Rising from the centre of the upper 



