ON THE LIMITS OF SINGLE VISION. 125 



point capable of transmitting the impression; in that of 

 Le Cat on the contrary, these points may occupy different 

 parts of the retina, which he calls the bottom of the eye. Experim^ntt, 

 Between authorities so respectable experiuients alone can 

 decide. Those to which I have had recourse are the most 

 simple: they consist chiefiy in producing an artificial squint- 

 ing, by changing the customary direction of the optic axis 

 by mechanical means. Every one knows, that such a. 

 change, when carried to a certain degree, will produce the 

 perception of a double image, which, being caused by a, 

 change in the respective situation of the points of the reti- 

 na simultaneously affected in ordinary cases, must be calcu- 

 lated to elucidate what relates to the place of the image. 



The firft result of the experiments by means of artificial Squlntm? 



squintinfr is, that the phenomena it exhibits are not recon- p'^?^'^*' ^*'** 

 ... .. It i-i not coil' 



cilable with tise opinion, in which the situation of the image fined ioasia« 



is confined to a single spot Bt the bottom of each eye ; for S'^ 'i^"^- 

 one of the impressions, bevng then necessarily made on a 

 part of the retina not intended to receive it, ought not to 

 give rise to any perception. From this single fact we may 

 conclude, that the place of the image is not necessarily at 

 the summit of the optic axis; but that several parts of the 

 surface of the retina are capabk^ like it of receiving the im- 

 pression, and producing perception. But we may assure 

 ourselves of the fact, by closing one eye, fixing the other 

 tvith a speculum, and then giving a lighted candle, first 

 placed in the line of the optic axis, various angular posi- 

 tions. Its flame will not cease to be visiole, till the anole 

 is at least 70°. It is true we do not see it perfectly, unlts.s 

 the angle be considerably less than this ; but we do not cease it extends oT?er 

 to perceive the flame, till its position is such, that none of •^^'i ^^''^'^j'*^ 

 the direct rays can reach the posterior hemisphere of the phere of' the 

 eye. It is evident therefore, that nature has not given the ^ye, 

 retina so great an extent in vain; and that the parts lyi no- 

 round the optic axis, though in a less favourable situation, 

 are equally capable of being affected. I will add, that, and objects ar« 

 having compared the impressions produced by rays parallel ^"°" ^'-^^ *" 

 with the optic axis, and others inclined to this axis, they ap- becaS mos/ 

 peared to me to differ in vividness only as far as would re- fa.vs can reach 

 wilt from the diminution of the apertare of the pupil occasi- "' 



oned 



