THE STRUCTURE AND FUNCTIONS OF THE EYE. 259 



by the common burning-glass. Concave lenses have a contrary 

 effect, causing the rays to diverge. 



The axis of a convex lens is exactly perpendicular to its two 

 surfaces, and this, in a double convex lens, passes through the 

 centre ; and when a ray of light comes in this direction, it passes 

 directly through without any refraction ; but an oblique ray is 

 turned inwards, so as to form a focal point with a ray coming in 

 a similar manner from the other side. 



These being the principal laws by which vision is governed, 

 we are enabled to understand the distinct offices of the various 

 parts of the eye. 



The transparent cornea, the window of the eye, serves to collect 

 the rays, and, by its convexity, to refract them towards the 

 ocular axis. 



The aqueous humour, besides its office of preserving the con- 

 vexity of the cornea and allowing the iris to float loosely amidst 

 it, serves likewise to refract the rays, from its being a denser 

 medium than the atmosphere. 



The cri/stalline lens, however, is the principal refracting glass ; 

 receiving the rays in some measure refracted by the cornea and 

 aqueous humour, it brings them to a focus on the retina. 



When either the cornea or the crystalline lens is too convex, 

 the object is brought to a focus too soon, and vision is indistinct, 

 and the person near-sighted. This is, probably, a frequent 

 defect in the eyes of horses that shy, in which we often find 

 full goggle eyes. When, however, the cornea or the lens is too 

 flat, a contrary defect is produced, and the object is not con- 

 verged soon enough. This is a frequent complaint in old 

 people, who are relieved by convex glasses, whilst the former 

 defect is remedied by concave lenses. 



The principal use of the vitreous humour is to preserve the 

 globular form of the eye, and thus afford a sufficient surface for 

 the spreading out of the retina, on which, as before observed, a 

 picture of external objects is painted, as may be seen by looking 

 into the eyes of a horse or any other animal. 



In taking this course, it must be evident that the rays which 

 enter at the upper part of the cornea are refracted at the lower 

 part of the retina, and vice versa. ; and thus objects are actually 

 represented in an inverted position, but, from being all painted 

 in the same relation to each other, they do not appear inverted 

 to the mind. — Ed.] 



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