NATURAL THEOLOGY. 59 



markable peculiarity in their eyes, which is required 

 by the nature of the element they inhabit. It is a 

 fact with respect to light, that whenever it passes 

 directly from water into a magnifying glass, the glass 

 must be more rounded, in order to produce the same 

 effect as would take place, if the glass was surrounded 

 by air. Agreeably, it is found, that the chief mag- 

 nifier in the eyes of those animals which inhabit the 

 water, is far more round than in those of land animals. 

 The structure which produces the tears is wanting in 

 fishes. The water in which they live renders the 

 tears unnecessary. The eel, which has to work its 

 way in the mud amid the hardest and roughest sub- 

 stances, is provided with a horny, transparent case 

 for the eye, which protects the part without ob- 

 structing the vision. The frog has a similar security. 



B. What, pray, can the frog want of a pair of 

 spectacles ? 



T. To defend the eyes from the sharp edges of 

 the spear-grass, &c., among which these little crea- 

 tures have to live. But you may give the descrip- 

 tion. 



A. As the progressive motion of this animal is 

 not by walking but by leaping, if his eyes were not 

 provided with such a case, he must either shut them, 

 and so leap blindfolded, or, by leaving them open, 

 must run the risk of having the front part of the eye 

 cut, pricked, or otherwise injured ; but this mem- 

 brane, like a kind of spectacle, covers the eye without 

 taking away the sight ; and, as soon as the occasion 



