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On Seeing in Water ; How do some Animals See in the Dark : 

 On Seeing at a Distance. 



1. On Seeing in Water. 



X HOSE animals whose eyes are organised 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 pro- 

 vided 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 sub- 

 merged, is provided on each side with two eyes, one pair situ- 

 ated on the crown of the head, for seeing in the air, and ano- 

 ther 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 ca- 

 pable of seeing in water with the one-half of the eye, and in air 

 with the other half. Thus Soemmering found in this fish, the 

 semidiameter of the upper cornea r=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 of it = 0.2 Paris lines. It can- 

 not be denied, that, in general, land animals can see under wa- 

 ter, and aquatic animals in air ; even man sees under water, al- 

 though the contrary has been maintained. It is not, however, 

 possible, that the same eye is ever so organised as to see equal- 

 ly 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 adapted for seeing equally well in 

 air and water, can have but imperfect vision in either. These 

 conclusions are in conformity \vith what is known of the power 

 of vision in those animals that live partly on the land and part- 

 ly in the water. The Seal (Phoca) is one of those animals that 

 live in both elements. But the seal has but imperfect vision in 

 the air. Rosenthal, in his memoir on the organs of the senses 

 of seals, says, " We have convinced ourselves by careful observa- 

 tion with living seals, of the species Phoca Grypus of Faber, that 

 the animal is always short-sighted in the air ; for when we held 



