288" ON THE OPTICAL PROPERTIES 



Refractive Powers. 



Sulphur, native, 



2.115 



Boracite, 



1.701 



Sulphuret of Carbon, 



1.680 



Tourmaline, 



1.668 



Blue topaz, 



1.636 



Flint glass, -i. 



1.616:. 



Dispersive Powers 



Oil of cassia, 



0.139 



Sulphur, 

 Phosphorus, 

 Sulphuret of Carbon, 

 Balsam of Tolu, 



0.130 

 0.128 

 0.115 

 0.103 



Flint glass, 



0.052 



Although oil of cassia surpasses the sulphuret of carbon in 

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

 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. All other fluids are sepa- 

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

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



yet 



