OF NATURAL HISTORY. 179 



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 cryftaliine, and ftretch in right lines towards its center. 

 When thefe fibres contra(3;, the diflance between the retina and 

 cryftaliine is lengthened ; and that diftance is Ihortened when thefe 

 fibres are in a relaxed ftate. The iris is that coloured circle which 

 furrounds the pupil. 



By this curious apparatus all the phaenomena'of vifion are con- 

 veyed to the mind. But, before we enter upon the manner in which 

 the different parts of the eye concur in tranfmitting the rays of light 

 and the images of objeds to the retina, it will be necefTary to give 

 fome general ideas concerning the nature of light, which is the uni- 

 verfal medium of vifion. 



Light confifts of Innumerable rays, which proceed in direct lines 

 from every part of luminous bodies. The motion of light, though 

 not inflantaneous, 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 diftance of the earth from the fun is fo immenfe, that, fup- 

 pofing a cannon ball to move at the rate of 500 feet in a fecond, it 

 could not come from the fun to the earth in lefs than 25 years. At 

 this rate, the velocity of light will be above 10 million of ' times 

 greater than that of a cannon ball. 



The rays of light, though they proceed in dired lines from lumi- 

 nous bodies, are refrafled, or bent out of their courfe, in pafTmg 

 through different mediums, as the air, glafs, and every tranfparenc 

 fubftances ; but, when they fall upon opaque bodies, they are reflec- 

 ted. Rays proceeding from any objedt, and paffing through a con- 

 vex glafs or lens, are refraded and colle£ted into a point, or fmall 

 I 'Za 1 fpace, 



