ISG THE PHILOSOPHY 



The globe of the eye is coinpofed of three humours, cal- 

 led aqueous, cryHalline, and vitreous ; and of the rciinn, ciliary 

 ligamcnty and iris. All thefe are contained within the/V^^ro- 

 tica and cornea, or capfule of the eye. The white part of the 

 cornea is opaque ; but the pupil, or7%/6/ of the eye, through 

 which the rays of light pafs, is tranfparent. The aqueous 

 humour is a menifcus, or a convex exteriorly, and concave in- 

 ternally. The crydallifie humour is doubly convex ; and its 

 exterior convexity is embraced by the concave furface of the 

 aqueous. The vitreous humour is likewife a menjfcus ; its 

 concave furface embraces the interior convexity of the cryf- 

 taUine, and its convex furface is encompalTed by the retina^ 

 >vhich is a fine expanfion of th^ medullary fibres of the optic 

 nerve fpread upon the convex furface of the vitreous humour, 

 and covering the bottom of the eye. The ciliary ligament is 

 a ring of fibres, which inclofe the edges of the cryflalline, and 

 ftretch in right lines towards its center. When thefe fibres 

 contrail, the diflance between the retina and cryftalline is 

 lengthened •, and that diftance is fhortened when thefe 

 ^bres are in a relaxed ftate. The iris is that coloured circle 

 fvhich furrounds the pupil. 



By this curious apparatus all the phaenomena of vifion are 

 ^conveyed to the mind. But, before we enter upon the man- 

 ner in which the different parts of the eye concur in tranf- 

 mitting the rays of light and the images of objects to the 

 retina, it will be neceffary to give fome general ideas conr 

 cerning the nature of light, which is the univerfal medium 

 of vifion. 



Light conHfts of innumerable rays, which proceed in di- 

 rect lines from every part of luminous bodies. The motion 

 pf light, though not inftantaneous, is inconceivably fwift. 

 To give fome comparative idea of its great velocity, it has 

 been difcovered by philofophers, that rays of light coming 

 from the fun reach this earth in feven minutes. Now, the 



