52 ANATOMY AND PHYSIOLOGY 



When fishes are out of water, they see indis- 

 tinctly, as a man descerns things with his head sub- 

 mersed. Pearl divers, on the coast of Brazil, see 

 objects precisely as an aged person does through 

 concave glasses. It has been ascertained that 

 there is an advantage in furnishing the divers with 

 spectacles, whose convexity, on both sides, is just 

 equal to the convexity of their own corneas. The 

 reason is plain : — without them they cannot judge 

 accurately of distances. In reaching out the hand 

 for an oyster, which appears within reach, it may, 

 perhaps, be at the distance of ten feet ; hence 

 there is a loss of time, requiring the Indian to come 

 up for breath, and plunge again, to correct the de- 

 ception of vision. The rationale of this error of 

 vision, is simply this : viz, the aqueous humor of 

 the diver's eye, is of the samedensity of the water 

 that covers him, therefore, there is no refraction of 

 the rays of light, or at least, only a very little, in 

 passing from the water, to the retina, but the con- 

 vex artificial lenses, remedy the defect, most per- 

 fectly. 



When the fish is brought into the air, the eye 

 not being defended either by eyelids, brushes of 

 hair, or, indeed, by anything more than the con- 

 junctiva, which is nothing more than a continua- 

 tion of the common skin of the head, carried over 

 the globe, the first glare of light partially paralyses 



