IMPROVED.- 113 



crillfd, when there are but few Rays of Light ; and 



the Circular Fibres contrail it when there are too 



many ; as may moft eafily be obferved in a Cat*s Obfervati- 



Eye when fhe fits in the. Sun-fhine, how naprow *^" °" '^^^^ 



and fmall the Pupil or Siglit of her Eye will ap- £yg 



pear to what it is when ihe is in the Shade. 



When the Pupil or Sight of the Eye is contra<^ed 

 we fee moll dillin^lly ; when it is dilated we fee 

 moil clearly. 



The Glaffy Humour keeps the Cryllalline at > 



fuch a Diftance from the Retina, as is necelTary for 

 uniting the Rays which come from one Point of the 

 Objeft exa£lly in one Point of the Retina. 



The Choroides is tinftured black, that the Rays The Choro 

 of Light which pafs through the Retina may ides black, 

 not be refleded back again, and thereby confufe^"d v;hy. 

 the Image of the Objeft ; for black imbibes or (as Black the 

 it were) fucks up the Rays, and therefore it J'JJJ^- ^^/'^ 

 is the hotteft Colour a Man can wear in Summer, summer- 



Thus have I given a fuccinft or Ihort Defcrip- wear, 

 tion of this admirably contrived Organ j and what 

 I fhall treat of next is the Theory of Vifion, 

 wherein I Ihall Ihew how and after what Manner, 

 the Images of Obje^ls are reprefented to the Eye, 

 by Means of the aforef?ii Organs of Sight. 



C H A r. XXL 



theory of Vifion. co^ifJeredy nvith fime of the Difcafe^ 

 of the Eye. 



IT is not my Defign at prefect to write a dif- 

 tin£l Treatife upon Opticks, but only to flievv 

 the Reader fo much as is necellary how Sight is 

 performed, and by that Means he will be able to 

 judge better, perhaps, than he could do before, of 

 the Soundnefs of a Horfe's Eyes. 



Light 



