ANIMAL MECHANISM. 133 



fish has but a drop, as it were, of aqueous humor, and, 

 moreover, the light arrives at its eyes through the whole 

 body of water above. The light is refracted only in a 

 small degree in entering its eye, because the humor is of 

 the same density of the fluid through which the light is 

 transmitted. The cornea is quite flat ; if it were promi- 

 nent, like the human eye, the sphere of vision would be 

 too circumscribed ; but by giving a prominence to the 

 whole, and placing the crystalline lens in the fore part 

 of the eye, they have a long diameter, and with the 

 provision of a large pupil, are completely fitted to see in 

 the element in which they are destined to live. With 

 an eye of this description, they must necessarily see in 

 air, as other animals see in water. 



Those animals whose eyes are organized for seeing in 

 water, see but indifferently in air. Hence, in those cases 

 where the habits of the animal require it to see in both 

 media, it is provided with two sets of eyes, or with eyes 

 accommodated for seeing in each element. Thus the 

 Gyrinus natator, an insect which generally swims on the 

 surface of water, but half submerged, is provided on each 

 side with two eyes, one pair situated on the crown of the 

 head, for seeing in the air, and another pair under the 

 head, for seeing in the water. It is also probable that 

 the fish named Cobitis anableps, which has in each eye 

 an upper and under cornea of different curvatures, and 

 for each cornea a particular anterior surface of the lens, 

 is capable of seeing in water with the one half of the eye, 

 and in air with the other half. Thus Scemmering found 

 in this fish, the semi-diameter of the upper cornea 1,0; 

 the under 1 ,2 ; the two curvatures of the upper part of 

 the lens 0,5 ; and the two curvatures of the under part 

 of it 0,2 Paris lines. It cannot be denied, that, in 

 general, land animals can see under water, and aquatic 

 animals in air ; even man sees under water, although the 

 contrary has been maintained. It is not, however, pos- 

 sible, that the same eye is ever so organized as to see 

 equally well in both elements. Land animals always see 

 indifferently in water, and aquatic animals imperfectly in 

 air. The one is long-sighted in water, and the other 

 short-sighted in air. An animal in which the eye is 

 VOL. i NO v. 12 



