J24 ^^ ^^^^ LIMITS OF SINGLE VISION. 



recited to the general laws of vision, by applying to them 

 the theory of corresponding^ points in the two retinas. These 

 points, and the limits of single v isioa, will form the subject 

 of this second paper. 

 Some con-f s- The slightest atteuion to the functions of the eyes must 

 iwmricncc be- long have taught physiologists, ihdt certain relations be- 



(t=!i-oei; parts of « i i i , • , „ 



C.etvroeyesto tween these organs are necessary forthe double impression of 

 ymduce siiigle one object to produce a single perception. The admirable 

 sympathy that prevails between the musclesi that move the 

 eyes, which constantly determines these organs to turn to 

 the same point; the difficulty of fixing one without the 

 other; and the almost absolute impossibility of moving 

 them simultaneously in different directions; would have 

 been sufficient to show, that these organs are formed to be 

 aftiected in concert, if the injuries affecting this arrangement, 

 and the »nost easy ocular demonstration did not present 

 themselves to confirm it. Accordingly they are unanimous 

 in acknowledging the necessity of this correspondence be- 

 tween the points of the retinas, which receive the impres- 

 sion of the double image at the same time. But in whut 

 does this correspondence consist ? can it be established be- 

 tween points taken throughout the whole extent of the reti- 

 na, between certain of these points, or between two of them 

 alone? These are questions on which authors are not agreed, 

 or which, not having been examined, appear to me to deserve 

 investigation. 

 lifaHer con- Haller, who in the 5th vol. of his Elements of PhysioTo- 



feiies the field gy ]^as collected what had been doiie by most of the learned 

 ef vision totlie 111 -.^ • • 1 ^- c 1 i • ic -^i ^ • 



•ptic axis. who had written envision, has satished himseir with laying 



down from Mariott's celebrated experiment the proposition, 

 thattheiniage cannotproduceany impression when received 

 on the optic nerve, but that it does when received on its outer 

 side at the point where the posterior extremity of the axis 

 of the bail of the eye, or optic axis, terminates; a part of 

 the retina, which he says, is supposed to be most sensible. 

 Le Cat extendi' Le Cat, perceiving the impossibility of limiting the position 

 jf '" ' '• of the image so strictly, has given the whole of the bottom 



of the eye for its field. Hence it follows, that in the opi- 

 nion of Mailer the points of correspondence are necessarily 

 fouiid ill tlie meetiiig of the optic axis, since this is (he only 



point 



