Mr. Gassiot on a Train of Eleven Sulphide-of -carbon Prisms. 69 



No. of cases of No. of cases of No. of cases of No. of cases of fa- 

 Year, facula to left facula to right facula equally on culae mostly be- 

 ofspot. of spot. both sides of spot, tween two spots. 



1858 2 



1859.. ..18 3 



1861.... 9 1 3 



1862.... 64 4 7 3 



1863.. ..47 9 2 



1864.. ..18 1 2 .., 1 



April 7. — Major- General Sabine, President, in the Chair. 



The following communication was read : — 



"Description of a train of Eleven Sulphide-of-Carbon Prisms 

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



The principles which should regulate the construction of a bat- 

 tery 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 angles of 45°*. 



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

 spectrum, flint glass stands unrivalled ; yet when the greatest amount 

 of dispersion is the desideratum, prisms filled with bisulphide of 

 carbon present obvious advantages, on account of the enormous dis- 

 persive power of that liquid — the difference of its indices of refrac- 

 tion 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 ce- 

 mented on with a mixture of glue and honey. This cement, on har- 

 dening, warps the sides, and confusion of the spectral lines is the 

 consequent result. To obviate this source of error, it has been pro- 

 posed 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 without any fear of derangement. 



On account of the lower refractive power of bisulphide of carbon, 

 as compared 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 ordinary 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 refractive and dispersive properties of crown glass and 



bisulphide of carbon, has substituted a prism of crown glass having 



* Phil. Mag., vol. xxvii. p, 143. 



