THE EYE. 



removed by lenses, composed of two species of glass, having diffe- 

 rent refracting powers, and whose curvatures are mutually adapted 

 according to principles established in optics. 



Now, it is a curious and highly interesting fact, that the eye, 

 which, as we know, is entirely free from this defect, owes its 

 perfection in this respect to the application of precisely the same 

 optical principle in its structure, so that if the first inventors of 

 the telescope had only thought of copying more closely the struc- 

 ture of the eye, they would have discovered sooner the principle 

 of ACHROMATISM, the name given to this precious quality of lenses, 

 from two Greek words, signifying the absence of colour. 



13. The structure of the eye being thus understood, it will bo 

 easy to explain the effect produced within it by luminous or illu- 

 minated objects placed before it. 



Let us suppose rays of light proceeding from any luminous 

 object, such as the sun, incident upon that part of the eye-ball 

 which is left uncovered by the open eye-lids. 



Those rays which fall upon the white of the eye, w, fig. 1 , 

 render visible that part of the eye-ball. Those rays which fall 

 upon the cornea pass through it. The exterior rays fall upon the 

 iris, by which they are reflected, and render it visible. The 

 internal rays pass through the pupil, are incident upon the crys- 

 talline, which, being transparent, is also penetrated by them, 

 from which they pass through the vitreous humour, and finally 

 reach the posterior surface of the inner part of the eye, where 

 they penetrate the transparent retina, and are received by the 

 black surface of the choroid, upon which they produce an illu- 

 minated spot. 



The aqueous humour being more dense than the external air, 

 and the surface of the cornea, which includes it, being convex, rays 

 passing from the air into it will be rendered by a general law of 

 optics more convergent, or less divergent. 



In like manner, the anterior surface of the crystalline lens 

 being convex, and that humour being more dense than tho 

 aqueous, a further convergent effect will be produced. 



Again, the posterior surface of the crystalline being convex 

 towards the vitreous humour, and this latter humour being lest 1 , 

 dense than the crystalline, another convergent effect will take 

 place. These rays passing successively through these three 

 humours, are rendered at each surface more and more convergent. 



14. If an object be placed before the eye, pencils of rays will 

 proceed from it, and penetrate the successive humours ; and if 

 these pencils be brought to a focus at the posterior surface, an 

 inverted image of the object will be formed there, exactly as it 

 would be formed by lenses composed of any transparent media 



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