178 T H E P H I L O S O P H Y 



exigence of objeds at comparatively fmall diftances ; and its infor- 

 mation is often imperfe(ft and fallacious. But the organ of fight, 

 which is mod admirably conftrudted, not only enables us to perceive 

 thoufands of obje<^s at one glance, together with their various fi- 

 gures, colours, and apparent pofitions, but, even when unarmed, to 

 form ideas of the flin and planets, and of many of the fixed ftars ; 

 and thus conne£ts.lis with bodies fo remote, that imagination is loft 

 when it attempts tfe form a conception of their immenfe magnitude 

 and diftances. Tils natural field of vifion, however great, has been 

 vaflly extended bf -the invention of optical inftruments. When 

 aided by the telefc^pe, the eye penetrates into regions of fpace, and 

 perceives flars innumerable, which, without the affiftance of art, 

 would to us have no exiftence. Our ideas of the beauty, magnitude, 

 and remotenefs or vicinity of external objedts, are chiefly derived 

 from this delicate and acute inftrument of fenfation. 



Before proceeding to the peculiarities of vifion, and the general 

 properties of light, we fhall give a fhort defcription of the ftrudlure 

 of the eye. 



The globe of the eye is compofed of three humours, called aque- 

 ous, cryftalline, and vitreous ; and of the retina, ciliary ligament, and 

 iris. All thefe are contained within the Jclerotica and cornea, or 

 capfule of the eye. The white part of the cornea is opaque ; but 

 the pupil, or fight of the eye, through which the rays of light pafs, 

 is tranfparent. The aqueous humour is a menifcxis, or a convex ex- 

 teriorly, and concave internally. The cryjlalline humour is doubly 

 convex ; and its exterior convexity is embraced by the concave fur- 

 face of the aqueous. The 'vitreous humour is likewife a menifcus ;. 

 its concave furface embraces the interior convexity of the cryftalline, 

 and its conve.x furface is encompafl^ed by the retina, which is a fine 

 sx^panfion of the medullary fibres of the optic nerve fpread upon the 



convex 



