Bi On Seeiiig in Water. , 



before it fish and other bodies, as pieces of wood or stones, it 

 did not distinguish them accurately, until they were brought so 

 near, that the organ of smell could be called into activity. 

 Scoresby remarks, " Whales are observed to discover one ano- 

 ther, in clear water, when under the surface, at an amazing dis- 

 tance. When at the surface, however, they do not see far."*' 

 Scoresby^s Arctic Regions^ vol. i. p. \<5Q. Faber, in his very 

 interesting work on the habits and manners of birds that inha- 

 bit high northern latitudes, (p. 298, § 65,) remark that Divers 

 (Colymbus) do not see so well above water as Grebes (Podi- 

 ceps), but better under water, because it is there they obtain 

 their food. 



It also appears, that birds which see well in one element, do 

 not see so well in the other. Faber proposes the question, " Is 

 it the case that divers, when under water, draw their nictitating 

 membrane over the eye, as they do when looking towards the 

 sun, in order to prevent the contact of the water ? It would 

 appear, from the observations of Treviranus, from whose excel- 

 lent work, entitled " Beitrage zur Anatomic und Physiologic 

 der Sinneswerkzeuge des Menschen und der Thiere, von Dr 

 G. R. Treviranus, fol. Bremen, 1828,'' the observations on 

 vision we are now detailing are principally extracted, that, by 

 drawing the nictitating membrane over the eye, divers, and 

 all other land animals, which seek their food under water, are 

 enabled, not only to prevent the immediate action of the water 

 on the eye, but also to discover their prey. But, as the light 

 loses more of its power on passing through water, than in pass- 

 ing through air, and is still more weakened in its progress 

 through the nictitating membrane, it follows, that, owing to this 

 membrane, vision must be less distinct under water than in the 

 air. 



2. How do some animals see in the dark f 



Is there any arrangement in the eye, and what is it, by which 

 animals that see in the dark are enabled to make up for the 

 want of external light ? When we consider the metallic lustre 

 of the tapetum, which in many animals occupies a great part 

 of the choroid coat, or even its whole surface ; further, its resem- 

 blance to a concave mirror, and its relation to the light that pe- 



