288" ON THE OPTICAL PROPEETIES 



Refractive Powers; 



Dispersive Powers; 



Although oil of cassia surpasses the sulphuret of carbon in 

 its power of dispersion; yet, from the yellow colour with whicb 

 it is always tinged, it is greatly inferior to the latter, as an op- 

 tical fluid, unless in cases where a very thin concave lens is re- 

 quired. The extreme volatility of the sulphuret is undoubted- 

 ly a disadvantage to which the oil is not liable ; but as this vo- 

 latility may be restrained, we have no hesitation in consider- 

 ing the sulphuret of carbon, as a fluid of great value in optical 

 researches, and which may yet be of incalculable service in the 

 construction of optical instruments. AH other fluids are sepa- 

 rated from these two, in their optical properties, by an immense 

 interval ; and hence we are of opinion, that oil of cassia will 



