SEEIxNG. 



objeftions not fo eafily repelled. Admitting the faft 

 rffpefting correfponding points to be true, it may be 

 obferved, that if we are taught hj feeling to fee objefts fingle, 

 notwithftanding a fenfation in each eye, the informations of 

 the former fenfe ought to be uniform, or elfe one fet of 

 yifual appearances would be aflbciated with diiferent reports 

 from feeling, and no certain mark would be afforded 

 us which of them we Ihould truft. Feeling, as Dr. Smith 

 allows, is not always the predominant, but fometimes the 

 inferior fenfe ; and its informations are not conftant and 

 original, but variable and derived. When a difference 

 occurs between the informations of the two fenfes, it is natural 

 to enquire what rule have we for determining which is moft 

 worthy of credit. But fuppofing that the fenfe of touch 

 to have originally and conftantly informed us that objects 

 are fingle, it would not follow that we are thence taught to 

 fee them alfo fingle. For, fince the place which an objefl 

 feems to either eye to poffefs, manifeftly depends both upon 

 its apparent diftance and its apparent direftion from that 

 eye, if vifible place be, in the language of Dr. Smith, 

 only an idea of real or tangible place, vifible direftion muft 

 bear the fame relation to tangible direction ; whence it fol- 

 lows, that we can never have a more accurate knowledge of 

 the direftion, in which an objedl may lie from any part of 

 our bodies, by fight than by touch. Whereas the contrary 

 is proved by fafts. Moreover whatever be the direction in 

 ■which an object may appear to either eye, it cannot be feen 

 in the fame place by both, except at fome point common 

 to the two directions. Accordingly Dr. Smith fays, that 

 when an objett is perceived fingle with both eyes, it is feen 

 at the mutual interfection of the two \ifual rays ; the vifible 

 direction of any object coinciding, according to him, with 

 the xafual ray, or the principal ray of the pencil which 

 flows from it to the eye. Should we then even allow, 

 that all we know by fight of the places of bodies has been 

 borrowed from feeling, it will dill be eafy to ftiew, that the 

 rule of vifion for each eye, which he has derived from fuch 

 experience, that of our feeing objefts in the direftions of 

 their vifual rays, is inconfiftent with many of the pheno- 

 mena of fight with two eyes ; and, confequently, that he has 

 left unremoved the chief difficulty of his fubjett, which w as to 

 explain the fingle appearance of objefts to both eyes, from 

 thofe laws, or rules of vifion, which affeft each of them 

 fingly. For it is a well-known fact, that if two bodies of 

 the fame fiiape, fize, and colour, be placed, one in each 

 optic axis, they appear but as one body, provided they be 

 at equal diftances from the eyes. Agreeably to the theory 

 of our feeing objedts in the direflion of their vifual rays, 

 this cannot happen, except the united body appear at the 

 interfection of the optic axes. Dr. Smith, accordingly, 

 maintains that it does. In the firll place, Dr. Wells appeals 

 to experiment for a direct proof that it does not ; and, in 

 the fecond, he obferves, that, as the two bodies in the optic 

 axes appear as one, whether they be fituated within or 

 beyond the concurrence of thofe lines, and as a right line 

 joining the bodies, and extended both ways, appears at the 

 fame time to the fight as a right line, it follows, upon 

 admitting the faft which he has denied, that all objects in 

 the plane of the optic axes which are feen in one pofition 

 and ftate of the eyes, however near to us, or however re- 

 mote they may in reality be, mult appear to be equally 

 diftant, or rather in a line drawn through the concourfe of 

 the optic axes, parallel to the interval between the eyes, 

 and named by opticians the horopter. Again, if a right 

 line be made to pafs through any part of the plane of the 

 optic axes, at right angles to it, the portions above and 

 below this plane are perceived to be in the fame right 



line with the point which is fituated in it, and the whole 

 appears perpendicular to the plane. But the point in the 

 plane is feen, by the laft article or propofition, in the 

 horopter ; the whole, therefore, of the perpendicular line 

 muft be feen in a plane palling through the horopter at 

 right angles to that of the optic axes ; or, in other words, 

 m the plane of the horopter, in which confequently all bodies 

 will hare their vifible places. But this was the very 

 opinion of Aguilonius, to which he was probably led by a 

 fimilar train of reafoning ; though, as a teacher, he might 

 choofe rather to ground it immediately upon an original 

 law of our conftitution. 



Dr. Reid agrees with Dr. Smith in his general principle, 

 but differs from him in maintaining the property to be ori- 

 ginal by which any two places in the retinas exhibit only 

 one object, while Dr. Smith derives it altogether from 

 cuftom. They differ alfo with refpedt to the meaning of a 

 term ; Dr. Smith calling correfponding points, fuch as have 

 the pofition juft mentioned, whether they reprefent objefts 

 fingle or not ; whereas Dr. Reid fays, that thofe points 

 correfpond, whatever their pofition may be, which reprefent 

 objefts fingle ; and he appears to Dr. Wells not always to 

 attend to the double ufe of the fame term, when he.fpeaks 

 of the opinions of Dr. Smith. 



Could it be fhewn, fays Dr. Wells, that the places of 

 the two retinas, which reprefent an objeft fingle when each 

 receives its pifture, are not the centres, or fuch others as 

 are fimilarly fituated, an obvious inference would be, that 

 the fingle appearance of the objeft is not occafioned by a 

 property in thofe places, beftowed upon them for this 

 fpecial purpofe by nature ; it being reafonable to expeft, 

 that fuch a property fhould be found, if any where, in 

 thofe parts of the retinas which are the moft like to each 

 other. 



Anatomifts have commonly taught, that the centres of 

 the fpheres, to which the cornea, the ball of the eye, and 

 the two portions of the cryftalline belong, are all placed 

 in the fame right line, hence called the optic axis, and that 

 this being produced both ways, paffes through the centres 

 of the cornea and retina, confidered as furfaces. Opticians, 

 on their part, obferve, that an objeft appears fingle to both 

 eyes, when the axis of each is accurately direfted to it ; 

 from which they infer, that the centres of the retinas agree 

 in fuggefting but one objeft, though each receives its pic- 

 ture. — Again ; fince it is known by experience, that, 

 while any objeft is feen fingle, to which the optic axes are 

 turned, others at the fame diftance from the eyes likewife 

 appear fo ; and fince the piftures of thefe lateral objefts 

 fall upon points in the two retinas, equidiftant from their 

 centres, and both upon the fame fide, that is, both to the 

 right or left of the centres, or both above or below them, 

 opticians conclude, that ever)- two places of the retinas, 

 which are fimilarly fituated with refpeft to the centres, 

 muft alfo agree in exhibiting but one objeft, though piftures 

 are received by both. 



But the whole of this reafoning, fays Dr. Wells, is 

 built upon a circumftance in the fabric of the eye, which 

 has been ftiewn by fome of the moft eminent anatomifts 

 not to have place. For Varolius long ago obferved, that 

 the cryftalline is not fituated in the middle of the eye, but 

 more in-jiardly ; and the accurate Zinn has more lately 

 mentioned, that if the eye be divided into a right and left 

 half, the centre of the cryftalline will be found in the inner 

 portion. Haller confirms this faft ; and Winflow's obfer- 

 vation, that the centres of the pupil and iris do not coincide, 

 but that the former is nearer to the nofe than the latter, is 

 connefted with it ; fince both Zinn and Haller agree, that 



the 



