308 Mr Herschers Experimenf on the 



Art. XVII. — Jccourit of an Experiment made on the composi- 

 tion of Oil of Cassia, to determine the cause of its high disper- 

 sive power ^* by J. W. F. Herschel, Esq. V.P.R.S. Sfc. <5c 



When the extraordinary dispersive power of Oil of Cassia 

 was discovered by Dr Brewster, he made the following obser- 

 vation on it: 



" The substances at the head of the table between the dis- 

 persive powers of 0.0128 and 0.400, (these numbers are values 

 of ^^J have never before been the subject of experiment, and 

 present us with results of unexpected magnitude. Chromate 

 of lead, realgar, and phosphorus, which are included within 

 these limits, might, from their chemical properties, be supposed 

 to possess a great degree of dispersion ; but the oil of cassia, 

 which exceeds even phosphorus in dispersive power, and stands 

 far above every mineral or vegetable product, exerts a most 

 surprising power in separating the extreme rays, and indicates 

 the existence of some ingredient which chemical analysis has not 

 been able to detect."" •\'\ 



After the same author \\2ididisco\eve&i\\2itSulphuret of Carbon 

 exceeded oil of cassia in refractive power, that of the former 

 being 1.68, and that of the latter only 1.64; while oil of 

 cassia exceeded the sulphuret in dispersive power, that of the 

 former oil being 0.139, and that of the sulphuret 0.1 15, he 

 remarks : — 



" All other fluids are separated from these two in their op- 

 tical properties by an immense interval ; and hence we are of 

 opinion, that oil of cassia will yet be found to consist of in- 

 gredients as remarkable as those which enter into the compo- 

 sition of sulphuret of carbon." J 



As the oil of cassia possesses also the remarkable property 

 of acting less powerfully upon green light than upon any other 

 substance yet known, § it became very interesting to determine 

 the principle to which such singular properties were owing. 



• From his Essay on IJifht, § 1 122. 



+ Treatise on New Philosophical Instruments, p. 310. 



X Edinburgh Transactions, vol. vii. p. 288. 



§ Id. vol. viii. p. 11. 



