90 EYES OF DIFFERENT ANIMALS. 



appear to be doubled in both its dimensions, and seem 

 consequently four times as large or it will have the 

 same appearance as if brought to half the distance. 

 There are three of those humours, as they are called, 

 in the eye of the more perfect animals. The aqueous 

 humour, which fills the foremost part of the eye, dis- 

 plays the iris or coloured portion that opens and shuts, 

 with the pupil or passage of the sight in the centre, and 

 it is supposed also to occupy a small portion behind 

 the iris. Behind the aqueous humour there is situated 

 the crystalline lens, which is equally transparent as the 

 aqueous humour, but of a firmer consistency, and has 

 both its sides convex or thickest at the middle. The 

 remaining part of the cavity is filled by the vitreous 

 humour, which is of a consistency between the two ; 

 and behind that, the retina or nervous tissue is spread 

 out, and supposed to be the most delicately sensible 

 part of the animal structure. 



Now it is in consequence of these lenses being of a 

 more dense structure than the substance to which their 

 convex sides are turned, that they cause the rays to 

 approach each other, magnifying the object, and render- 

 ing it more distinct. The front surface of the aqueous 

 humour refracts the rays that come through the less 

 dense air, and they are further refracted by both sur- 

 faces of the crystalline lens. But animals, that live in 

 water, and receive the rays of light through that 

 medium, would not have them brought together by an 

 aqueous humour : and, therefore, the external eye in 

 fishes is nearly flat, while the convexity of the crystal- 

 line is increased till it be almost a little globe, like one 

 of the most powerful single-lens microscopes. 



