42 



Proceedings of the Royal Irish Academy. 



YI. — Ox A SpECTEOSCOPE of the BrN'OCriAB FoEM foe the OBSEETATIOiy 



or Eai^'t Spectea. Ey Chaeles E. Etjeiox, B. A., F. E. A. S. 



[Read May 25, 187-i.] 



The instrument described in this notice was constructed at the in- 

 stance of Mr. Stoney, out of a grant of money entrusted to me for the 

 pui'pose by the Eoyal Irish Academy. Owing to the pressui'e of other 

 business upon ITr. Grubb and ITr. Spencer, on whom deTolved the 

 carrying out of the details of the construction, the instrument has 

 only come into my hands within the last few days, and its capabilities 

 must, therefore, be considered as not yet fully tested. 



The binocular form was adopted at Air. Stoney' s suggestion, in 

 order to secure that great increase of power and comfort in observa- 

 tion of faint and difficult objects which results from the co-operation of 

 both eyes. I append an outline sketch of the spectroscope, of which 

 the following is a description. 



The same parts are designated by the same letters in both plan 

 (Fig. 1.) and elevation (Fig. 2.), and are drawn to the same scale, 

 namely, one-sixth of full size. The working parts are sustained and 

 connected by a mahogany frame-work B, which protects the internal 

 arrangement, and gives support to the recording table, marked Slider. 



Collimators. The collimators C C, which carry the slit -S' are 

 fixed upon a mahogany base-board, movable in a vertical plane about 

 the axis a a', and capable of being fixed at any required inclination 

 to the line h h by means of a binding screw s, which passes through a 

 circular slot in B into the wooden base-piece. 



The Prisms. P and P' are the prisms, worked as nearly as possible 

 to the same form. The three faces of each prism are polished, so that 

 any one of the three angles may be employed to form a spectrum. 

 In each prism the three angles are not equal, but subtend respectively 

 58°, 60°, and 62°. The range of dispersion obtainable with a single 

 prism is, therefore, very considerable, if the three angles are made 

 use of in succession. 



