Chap. 41-3 S**fi of 'Sight. 393 



to tlie animal in its peculiar manner of life from this 

 Angularity *.' 



It was neceffary, in a former part of this work, to 

 notice the fubjecl: of vifion, in defcribing the effects 

 and phenomena of light f. The eye was then men- 

 tioned as a mere optical inftrument, but after the par- 

 ticular defcription of that organ, which has now been 

 given, a more particular inveftigatian of the ienfe of 

 fight feems to be required ; and ihould the reader find 

 any thing like repetition in what \\t-ill now be fubmitted 

 to him, his candour will, I doubt not, pronounce my 

 apology for endeavouring to render as clear as pofiible 

 a fubjeet which is at once both important and difficult 

 to be underftood. 



It has been fumdently explained, that from every 

 point of a vifible object the rays, or rather pencils, 

 of light are emitted or reflected in every direction j 

 t>ut to produce vifion, it is neceffary that they fhould 

 be concentrated or converged to a fuch point as to make 

 a forcible impreffion on the retrna. Thus from the 

 luminous body A (Fig. 6.) the rays r } r, r are fent in 

 various directions. Thofe which fail upon the tranf- 

 parent cornea C C are there refracted in fuch a manner 

 as to enter the pupil at p, and in pafiing the cryftalline 

 lens or humour they fuffer a fecond refraction, and are 

 converged to a point or focus at the point a on the re- 

 tina. Now it is evident, that if the rays could have 

 pafled the humours of the eye in their natural direction, 

 that is in the direction of the cone or pyramid C, A, C, 

 they would have made upon the retina a very exten- 

 sive but feeble impreffion, fuch as we know by expe- 

 rience could not produce diftinct vifion ; to obviate this 



* Monro's Comparative Anatomy, 

 f -See book iii. chap, 7. 



