1864.] Mr. Gassiot — Prisms foi^ Specti^um Analysis. 183 



have been unproductive of good, as well on account of the confirmation 

 they afford of the truth of the rule, towards the establishment of which on 

 scientific grounds they constitute the first serious step yet made, as also, 

 and still more, by reason of the accessions to the existing field of algebraical 

 speculation to which they have incidentally led. 



III. ^^Description of a train of Eleven Sulphide-of-Carbon Prisms 

 arranged for Spectrum Analysis.^^ By J. P. Gassiot^ F.R.S. 

 Received March 17, 1864. 



The principles which should regulate the construction of a battery of 

 prisms have been alluded to in the description of the large spectroscope 

 now at Kew Observatory, which has a train of nine dense glass prisms with 

 refracting ano;les of 45°*. 



While for purposes of exactitude, such as mapping out the solar spec- 

 trum, flint glass stands unrivalled ; yet when the greatest amount of dis- 

 persion is the desideratum, prisms filled with bisulphide of carbon present 

 obvious advantages, on account of the enormous dispersive power of that 

 liquid — the difference of its indices of refraction for extreme rays being, 

 according to Sir David Brewster, as 0'077 against 0*026 for flint glass. 



In the fluid prisms of the ordinary construction, the sides are cemented 

 on with a mixture of glue and honey. This cement, on hardening, warps 

 the sides, and confusion of the spectral lines is the consequent result. To 

 obviate this source of error, it has been proposed to attach an additional 

 pair of parallel sides to such prisms, a thin film of castor-oil being interposed 

 between the surfaces. The outer plates are then secured by means of 

 sealing-wax, or some cement, at the corners. In the battery of prisms now 

 about to be described, Mr. Browning has dispensed with this attachment at 

 the corners, which is likely to prove prejudicial, and has secured the second 

 sides in their proper position by extremely light metal frames which clasp 

 the plates only on their edges. 



Thus arranged, the frames exert no pressure on the surfaces of the 

 plates, and are quite out of the field of view, and they can be handled with- 

 out any fear of derangement. 



On account of the lower refractive power of bisulphide of carbon, as com- 

 pared with flint glass, a refractive angle of 50° was given to the fluid prisms. 

 Eight such prisms would cause a ray of light to travel more than a circle, 

 and would be the greatest number that could be employed had the ordi- 

 nary arrangement been adopted. 



In place, however, of giving to the fluid prisms two pairs of parallel 

 sides, Mr. Browning, taking advantage of the difference between the re- 

 fractive and dispersive properties of crown glass and bisulphide of carbon, 

 has substituted a prism of crown glass having a refracting angle of 6° for 

 one of the outer plates of each prism — the base of this crown-glass prism 

 being brought to correspond with the apex of the fluid prism, thus : — 

 * Proceedings, vol. xii. p, 536. 



