J94 ON THE OPTICAL PROPERTIES 



the two images ; the acliromatic 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 

 poners 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 : 



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, -w^iich are extremely near 

 to each other, we obtain for the 



Least refractive power, - 1.3350 



Greatest refractive power, - 1.5145 



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

 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 

 accessary to confirm it by repeated observations. 



la 



