1 16 ^he Aft 0/ Farriery 



the Rays converge or come to a proper 'Focus cr 

 Point, though they were before fpread in a large 

 and confufed manner j that is, the oblique Rays of 

 each Pencil of Rays, that flows from every Point of 

 the Objeft, will be refradled by the Denfity of the 

 Glafs Medium to their proper Perpendicular, and 

 there meeting much nearer to one another in the 

 Focus, than at their Place of Incidence, muft con- 

 fequently imprint the Image much more Ilrongly, 

 and perfeftly delineated. 



If the Lens or Cryftalline Humour be too far from 

 the Paper or Retina, the Rays will interfeft or 

 crofs one another before they arrive there, and after- 

 wards become divergent or difperfed upon the Pa- 

 per; fo that by this Means they are again fpread, 

 and make a confufed or faint Image or Impreffion 

 of the Objed. If it be held too near, I mean, if 

 the Lens or Convex Glafs be held too near the Pa- 

 per, the Rays are not as yet met in their Focus, and 

 therefore cannot be {o perfeft. Thus the Paper or 

 Retina muft b« at a proper Diftance. 

 j^ear- The Reafon commonly afTigned for the Diforder 



fightednefs of Sight called Myopia, Moufe-ey'd or Purblind, is 

 accounted ^his, to wit, from too great a Convexity or Protu- 

 berancy of the Cornea and Globe of the Eye, which 

 is very remarkable in Creatures near-fighted. There 

 is no Cure for this Diforder in Horfes, but Men are 

 help'd by the Ufe of Concave Spe6lacles or Menif- 

 cous Glaffes, .or fuch Glaffes as are Convex on one 

 Side and Concave on the other, or thofe called Con- 

 cavo-convex Glaffes, which make the Rays diverge 

 or go farther afunder, that otherwife v/ould be too 

 near or Convergent, by the too great Convexity of 

 the Cornea, i^c, 

 presbytia, Prefaytia, from 'Tr^'iffCv;, Senex, old, i.^ a Dif- 

 what, temper juft oppofite to the former ; and is when the 



Cornea, t^c. are not fo Convex, as they were in 

 In old Youth : For, as any Creature advances in Years, 

 H<>rfcs,the the Eye grows more plain and flat, for want of that 



Cornea not yiprour in the Blood's Impulfe, which was wont to 

 fo Convex 6 J ^^^^ 



