125 ON THE LIMITS OF SINGLE VISION. 



oned by the obliquity of its pbnie t<» the luminous rays, and 



by the oblitpiity of these rays to tiie refnictive meijiu. 



All parts of the It is true the vividness of the images situate at the optic 



retina aie pro- pf,]j^ pyj^ ^^^ Compared witli that of those around it only bv 



babljr equally ' . . , ■ . r,- • . i • i- re " 



sensible: estimation; but this is sumcient to prove, that their diner- 



ence is not such as it uoald be, if it were owing to n dimi- 

 nution of sensibility in the parts of the retina remote frovn 

 the optic pole, and that consequently the retina is not de- 

 prived of tiie faculty of transmitting the ^impressions re- 

 ceived on those parts ; whence the points of correspondence 

 are not necessarily and invariably situate in the optic axis, 

 as several authors have supposed. I wouUi not venture to 

 ' assert, that all the parts of the retina, on which images may 

 be painted, are equally sensible^ the experiments I have 



at lea>t there is made not being incontestibly decisive of this: but they 



no great (litter- pj.^^g ^^ least, that the diiference is not very sreat at a cer- 

 ence lo some r _ ■ -' » 



distance louHc! tain distance from the optic axis; for, notwithstanding the 

 iheaxis. influence of the causes before mentioned, the light of a 



candle received into the two eyes, inclining them sufficiently 

 for the images to make with the optic axis angles of 15", 

 20'', and 25", did not exhibit any diminution of lustre, that 

 could be appreciated. These facts, which may easily be ve- 

 rified, give to the field of distinct vision, and to that of per- 

 fect vision, a much more considerable extent than is assigned 

 Dr. Young. to them by Dr. Young in a learned paper on vision, an 

 abridgment of which is given in the J 8th vol. of the BiUio~ 

 ilitque Britannique. 

 The image The point that corresponds with the optic axis, without 



mos! pettectin \^f.\y^^ endued with sensibility superior to that of other parts 

 ihe optic axiif. „ , . . , , „ r . ■ ■ i •, - • 



of the retina, is the place ot perfect vision,, because it is in 



the focus of the refraclive apparatus, the only point where 

 the image can have its whole perfection. The muscles, de- 

 termined by habit to dispose the ball of the eye so as to re- 

 ceive the most vivid impression, bring it into this position, 

 because it is the most advantageous, and not because it is 

 the only one in which vision can take place; which is equally 

 proved by artificial sciuintiug, and by that accidental squint- 

 Chcseiflen. ing which Cheselden mentions. 



The corres- The Optic poles are most commonly the place of the <lou- 



poudiiigpoii.t. i^jg image, or points of correspoudeuce. These parts how- 

 ever 



