Chap. ;.] [ 241 ] 



CHAP; VII. 



OF VISION. 



fhe Eye defcriled as an optical InJlrument.Defefls of the Eye. Short 

 and weak Sight. The former remedied by double concave, and the 

 /after by double convex, Glaff'es. How the Faculty of feeing correSl - 

 ly is acquired. Apparent Magnitude depends on the Diftance ofQ{f> 

 jefls.Pifual Jingle. Phenomena of ViJion.^Why the d+fta**^ 

 Parts of an horizontal Plain, and of the Sea, appear elevated. 

 Why a long Watt appears curved.* Why a high Tower appear! 

 falling to an Eye beneath, Belt of Saturn. Phenomena of Pi/ion at 

 tonnefted with Motion. Why the Moon appears to move in/lead of 

 the Clouds ivhich pafs over its Face. Why a Circle viewed ob- 

 liquely appears oval. How GlaJJts ajjijl the Sight. 



TH E ftructure of the eye will be confidered irt 

 another place * ; it will be fufficient therefore at 

 prefent to treat it as a fimple lens, which has the 

 power of refracting the rays of light, incident upon itj 

 on the retina, whence we derive all our ideas of fight, 

 and by repeated experience correct the errors which 

 by that organ alone might have been produced. Let 

 A B (Plate XX. Fig.J9.) rcprefent a fection of the eye> 

 MN being the pupil, AqpB the retina or optic nerve 

 expanded over the internal furface of the eye, and let 

 qp be the image of QP made by the lens M N. The 

 action produced on the part of the retina q.p occafions 

 a fenfation, from which we derive by the fight our 

 knowledge of external objects. 



Now if the lens M N was fixed, the image of two 

 objects at different diftances from the eye would not 

 fall on the retina, and therefore it is wifely provided 



* Book IX. Chap. 41. Senfe of Sight, 



VOL, I. R by 



