294 GN THE OPTICAL PROPERTIES 



the two images ; the achromatic nebulosity of the first, and the 

 distinctness and deep colours of the second image, formed al- 

 together a singular phenomenon, and, at the same time, afford- 

 ed an ocular demonstration of the existence of two dispersive 

 powers in doubly refracting crystals. 



The following measures of the refractive powers, of the two 

 images were taken with the greatest care : 



Angle of the prism, - 60° 21/ 



Angle of refraction for the 1st image, 24° 8' 

 Angle of refraction for the 2d image, 38° 54' 

 Index of refraction for the 1st image, 1.3374 

 Index of refraction for the 2d image, 1.5156 



In order to confirm these results, I formed a new prism, and 

 obtained the following measures i 



Angle of the prism, - 62° 12' 



Angle of refraction for the 1st image^ 24° 48' 

 Angle of refraction for the 2d image, 40° 39' 

 Index of refraction for the 1st image, 1.3326 

 Index of refraction for the 2d image, 1.5134 



By taking a mean of these results, which are extremely near 

 to each other, we obtain for the 



Least refractive power, - 1.335Q 



Greatest refractive power, - 1.5145 



Hence it follows, that the least refraction of nitrate oTpot- 

 ash, is almost exactly the same as that of Water, which is 

 1.3358, — a result of such an extraordinary nature, that I felt it 

 necessary to confirm it by repeated observations, 



la 



