ON THE T II O U T. 



looks over the edge of a bank, D, in the direction 

 A F Z, he might (if unacquainted with the laws of 

 refraction) imagine, that neither the fish C, nor 

 any other fish below the line of his direct vision, 

 AFZ, could see him; whereas C could see A B 

 by means of the pencil of light, AFCEB, bent, 

 or refracted at the surface of the water, E F, and 

 the image of A B would appear in the eye of the 

 fish shortened and transferred to G H. The fish 

 in fact could see the whole of the man, round, or 

 over the corner of the bank, by the aid of the water 

 above C, if both were situated as respectively 

 represented in the diagram ; but if the surface of 

 the water should be at IK, (i. e.) about as low as 

 the fishes' eye, then, he could not see any part of 

 the figure AB, because a straight or unrefracted 

 pencil of light, ACB, would be obstructed by 

 the bank. 



Increments of obliquity in pencils of light fall- 

 ing upon a surface of water, &c. are accompanied 

 by increments of refraction, not in direct ratio to 

 the increase of obliquity, but in a much higher 

 ratio ; and indistinctness of vision in an eye re- 

 ceiving the pencil increases, on this account, in 

 some similar high ratio. 



The bending or refraction which a pencil of 

 light, as NEOFM, (fig. 2), falling very obliquely 

 upon the surface of the water, undergoes before 



