120 On the Optical Properties 



A prism of flint glass; with a refracting angle of 20" 53', cor- 

 jects the colour produced by a prism of sulphuret of carbon, 

 having a refracting angle of 8"^ 10'; the light being incident 

 perpendicularly upon the fluid prism. Hence it follows, that 



the dispersive power of the sulphuret, or the value of — — - 



is 0" 115, R being the index of refraction, and d the part of the 

 whole refraction to which the dispersion is equal ; and that the 

 refractive power of the extreme red ray is 1*623, and the re- 

 fractive power of the extreme blue ray l-7'>7. 



From these experiments we conclude, that the sulphuret of 

 carbon exceeds all fluid bodies in refractive power, surpassing 

 even flint-glass, topaz and tourmaline ; and that in dispersive 

 poti'er, it exceeds every fluid substance, except oil of cassia, 

 holding an intermediate place Jietiveen phosphorus and balsam 

 of Tolu. 



These relations will be better understood from the following 

 short tables : 



Refractive Powers. 



Sulphur, native 2-115 



Boracite 1701 



Sulphuret of carbon 1*680 



Tourmaline 1 *668 



Blue topaz 1 -636 



Flint glass 1*616 



Dispersive Powers. 



Oil of cassia 0*139 



Sulphur 0*130 



Phosphorus 0*128 



Sulphuret of carbon 0* 1 15 



Balsam of Tolu 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 optical 

 fluid, unless in cases where a very thin concave lens is required. 

 The extreme volatility of the sulphuret is undoubtedly a disad- 

 vantage to wliich the oil is not liable ; but as this volatility may 

 be restrained, we have no hesitation in considering 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 separated from these 

 two, in their optical properties, bv an immense interval ; and 

 hence we are of c pinion, that oil of cassia will yet be found to 

 consist of ingredients as remarkable as those which enter into 

 the composition of sulphuret of carbon. 



IL Car- 



