SEEING. 



the centre of the pupil is placed in the axis of the cryftalhne, 

 while that of tlie iris is evidently in the common axis of 

 the cornea and globe. Now, a confequence of this pofition 

 of the cryftalline is, that, contrary to what is univerfally 

 maintained, no ray of light whatfoever can pafs unbent to 

 the retina from the atmofpherc, or any other medium 

 differing in refradive power from the aqueous humour. 

 If, then, the line joining the centres of the cornea and 

 globe of the eye be what is called the optic axis, and if it 

 be true, that objetts appear fingle when we diredl both 

 thefe axes to them, it muft be evident, to fuch as are ac- 

 quainted with the common rules of optics, that the piftures 

 of thofe objtcls do not fall upon the centres of the retinas, 

 but more internally ; and, therefore, that the centres and 

 all the other points of thofe membranes, which by the 

 prefent fyftem are fuppofed to reprefent objects fingle, do 

 in faft exhibit them double. 



Admitting, however, that objefts are reprefented fingle, 

 when their pifturcs fall upon the centres of the retinas, or 

 upon any other two points which are equally diilant from the 

 centres, andboth upon the fame fide, it appears to Dr. Wells, 

 notwithflanding, to be in violation of all analogy, to afcribe 

 this effeft, with refpeft to the points at lead, on the right 

 and left fides of the centres, to any peculiar property which 

 they poffefs from nature. For when anatomifls find, in a 

 new fpecies of animals, organs fimilar in ftrufture to thofe 

 of others they are already acquainted with, they immediately 

 conclude, that they are alfo fimilar in regard to their ufe. 

 In animals of the fame fpecies, they believe with certainty, 

 that the organs they fee in one have the fame properties as 

 the correfponding organs of another ; and, it it be pof- 

 fible, they attribute with greater certainty the lame pro- 

 perties to two organs of the like kind, which ai-e found in 

 the fame individual. Such is the influence of the rule, that 

 refemblance of property is implied by refemblance of i\ruc- 

 ture. Now it is an univerfal faft, that if an animal be 

 divided into a right and left half, the correfponding parts 

 of thofe organs which exift in pairs are found at equal 

 difl;anccs from the plane of partition. Thus, for inllance, 

 in refpeft to the eyes, the two optic nerves penetrate their 

 outward coat at the fame diflance from this plane. Their 

 mufcles, blcod-veflels, and every other of their component 

 parts and appendages, are arranged in the like manner ; 

 thofe nearefl: to the dividing plane, or the innermolt, in the 

 one, being fimilar in ftrufture to the innermoll in the other, 

 the outermoft; to the outcrmoft, and the intermediate to the 

 intermediate. It is furely, therefore, natural to expeft, 

 that fuch parts fhould alio be fimilar in their properties ; 

 and we in faft find this fimilarity to exift, wherever it can 

 be clearly afcertainjd what the properties are. Every 

 perfon, for example, admits, that the internal ftraight mufcle 

 of the right eye performs the fame office, with refpeft to 

 that eye, as the other internal ftraight mufcle does with 

 refpeft to the left eye. What judgment are we then to 

 form of the opinion of Dr. Reid, which attributes the fame 

 original properties, or rather the joint pofleffion of one 

 original property, to places in the retinas fituated at unequal 

 diftances from the general plane of partition ; which makes 

 an external point in one to correfpond, in ufe, with an 

 internal point in the other, and this too by a principle 

 implanted by nature ? If fuch things exift, they may, at 

 leaft, be faid to ftand oppofed to a moil extenfive analoo-y. 



To thefe arguments, a priori, againft the opinion of 

 Dr. Reid, Dr. Wells adds others derived from a confider- 

 ation of its confequences : but our limits oblige us to refer 

 for thefe to the author's own account. If objefts, it may 



be faid, appear fingle neither from cuftom, nor from an 

 original property of the eyes, have we not an effeft without 

 a caufe, and muft there not be fomething wrong in the 

 fafts or reafoning which lead to fuch a conclufion ? Dr. 

 Wells replies : Since vifible place contains in it both 

 vifible diftance and vifible direftion, it is not neceffary that 

 the fingle appearance of an objeft to both eyes, ftiould 

 depend altogether either upon cuftom, or an original 

 principle of our conftitution ; for its vifible diftance to 

 each eye may be learned from fcelmg, and its vifible di- 

 reftion be given by nature ; in which cafe, the unity of 

 its place to the two eyes, will be owing to neither of thofe 

 caufcs fingly, but to a combination of both ; and this Dr. 

 Wells regards as a fufficient reply. 



Dr. Wells proceeds to propofe and illuftrate his ov.'n 

 theory for the folution of the queftion, why objefts ai-e 

 feen fingle with two eyes ; or, in other words, why they 

 appear in the fame place to both ? The vifible place of an 

 objeft being compofed, as he conceives, of its vifible dif- 

 tance and vifible direftion, it becomes neceffary, for fliewing 

 how it may appear the fame to both eyes, to explain in 

 what mannner the diftance and direftion, which are per- 

 ceived by one eye, may coincide with thofe which are per- 

 ceived by the other. He begins with a confideration of the 

 diflance. In judging of diftance by fight, no perfon has 

 ever obferved, that while an objeft feemed to one of his eyes 

 at a certain diftance, it has appeared to the other to be at 

 a different diftance, and from this circumftance alone has 

 been feen double ; or, to exprefs the fame thing in another 

 way, that while the vifible appearance of an objeft to one eye, 

 covered the vifible appearance of the fame objeft to the other 

 eye, the two appearances did not feem entirely to coincide, 

 and make one, but were feen feparate by the two eyes. Hence 

 it follows, that the difficulty in finding a true and fufficient 

 caufe for the union of the two vifible places of one or two 

 objefts to two eyes, muft therefore confill altogether in 

 fhewing, in what manner the two apparent du'cclions may- 

 coincide, confiftently with the attending phenomena. 



From the time of Kepler's difcovery of the feat and 

 manner of vifion, there have been, fays our author, only 

 two theories offered refpefting the apparent direftions of 

 objefts. One is, that they are perceived in the direftion of 

 lines pafling from their piftures on the retina, through the 

 centre of the eye ; the other, that their apparent direftions 

 coincide with their vifual rays. But both of thefe theories 

 are inconfiftent with the phenomena of fingle vifion with 

 two eyes. For according to neither of them can an objeft, 

 placed at the concourfe of the optic axes, be feen fingle, 

 unlefs we have a moft accurate knowledge of its diflance ; 

 nor will either admit two objefts to be feen as one, which 

 are fituated in the optic axes, whether on this fide, or beyond 

 where they meet, unlefs the united objeft be referred by 

 figlit to their very point of interfeftion ; both of which con- 

 clufions are contradifted by experience. It is evident, 

 therefore, that fome other theory of vifible direftion is 

 required, which fhall not be liable to thefe objeftions. 



Dr. Wells's theory is illuftrated in the following propo- 

 fitions, which we can merely flate without enlargement : 

 I. Objects fituated in the optic axis do not appear to be ia 

 that line, but in the common axis ; i.e. in a line drawn 

 from the middle of the vifual bafe, through the point of 

 interfeftion of the optic axes, or parallel to them, if they 

 be parallel to each other. 2. Objefts, fituated in the com- 

 mon axis, do not appear to be in that line, but in the axis of 

 the eye, by which they are feen. 3. Objefts, fituated in 

 any line drawn through the mutual interfeftion of the 



optic 



I 



