crystallized, surfaces upon light. 
*47 
light, or almost invisible. A result nearly similar will be 
obtained with water, notwithstanding its effect in dissolving 
the little prominences which constitute the superficial rough- 
ness of the crystal. 
In order to explain these phenomena, we must recollect that 
the index of refraction for the ordinary image of nitre is 1.511, 
and that of the extraordinary image 1.328. When the rough 
surface of the nitre is covered with balsam of capivi, which 
has nearly the same index of refraction as the ordinary 
image, the same effect is produced as if the rough surface 
had been polished for the ordinary rays. All the little pits 
or depressions in the rough surface being filled up with 
balsam, the ordinary rays suffer little or no refraction in 
penetrating the crystal, and therefore the image which they 
form will be as clear and distinct as in the first experiment. 
But since the index of refraction for the extraordinary image 
is much less than that of the balsam, the rays of which it is 
composed will not enter the crystal undisturbed, but will be 
scattered in the same manner as if its surface was rough, and 
had a refractive power corresponding to the difference between 
the index of refraction for the extraordinary ray, and the 
index of refraction for the balsam. When water or alcohol 
is substituted in room of the balsam, the effects now described 
are interchanged, the roughness being removed for the extra- 
ordinary rays by the application of a fluid of the same refrac- 
tive density, while the rays that form the ordinary image are 
dispersed by the refractions which still exist at the rough 
surface of the crystal. 
These effects will be still better understood by supposing 
the crystal to consist of an extraordinary and an ordinary 
