VISION. 



1447 



object the mind, by virtue of a connate immu- 

 table law, traces back its own sensation from 

 the sensorium to the retina, and from thence 

 outwards, along right lines drawn perpen- 

 dicularly from every point of the retina on 

 which any impression is made by the rays 

 forming the picture, towards the object itself, 

 by which means the mind always sees every 

 point of the object, not in the sensorium or 

 retina, but without the eye, in these perpen- 

 dicular lines. But these lines nearly coincide 

 with the axes of the several pencils of rays 

 that flow to the eye from the several points of 

 the object ; and since the mind has also a 

 power of judging rightly of the distance of 

 objects, it follows that every point of the 

 object must appear and be seen in the place 

 where it is, and consequently the object must 

 appear in its true erect position, notwithstand- 

 ing its picture on the retina is inverted. This 

 theory of lines of visible direction Reid regards 

 as a law of nature, of which our seeing objects 

 erect, from inverted images, is a necessary 

 consequence. Sir David Brewster too believes 

 it erect vision results from the lines of 

 risible direction being in all cases perpen- 

 licular to the impressed part of the retina ; 

 it Miiller offers the following objections: 

 The hypothesis that erect vision is the re- 

 ilt of our perceiving, not the image on the 

 3tina, but the direction of the rays of light 

 ,'hich produce it, involves an impossibility, 

 since each point of the image is not formed by 

 rays having one determinate direction, but by 

 an entire cone of rays, And, moreover, vision 

 can consist only in the perception of the state 

 of the retina itself, and not of any thing lying 

 in front of it in the external world. The 

 hypothesis also that the retina has an outward 

 action, and that objects are seen in the direc- 

 tion of decussating lines, that is to say, in the 

 direction of the perpendicular of each point of 

 the concavity of the retina, is a perfectly arbi- 

 trary assumption ; since there is no apparent 

 reason why one direction should have the pre- 

 ference rather than another, and each ultimate 

 sensitive division of the retina, if it had the 

 power of action beyond itself, would act in as 

 many directions, as radii might be drawn from 

 it towards the exterior world." * Notwith- 

 standing these objections, the law of visible 

 direction affords the most satisfactory expla- 

 nation of the phenonmena of erect vision ; all, 

 however, we know positively is, that in the or- 

 dinary exercise of vision, the mind infers the 

 positions of objects from an impression made 

 upon the retina, and that it as certainly draws 

 the right conclusion therefrom. 



This, however, is not more wonderful than 

 that the undulations of the luminous particles 

 should excite the sensation of light in the 

 retina, or that vibrations acting on the auditory 

 nerve should give rise to sound. We may 

 heap conjecture on conjecture as to the final 

 cause of these phenomena, but we must stop 

 at the limits to the boundaries of human 

 knowledge. The profound resources of the 



Divine Intelligence excite our wonder and 

 exalt our thoughts, but there are a thousand 

 things abstractedly possible which set at 

 nought our comprehension. 



It is curious that such an acute mind as 

 that of the late eminent metaphysician, Dr. 

 Brown, should have been so satisfied that the 

 perception of the number and position of 

 visible objects is acquired only by association 

 or custom, that he dismissed the subject thus 

 curtly: "In the single vision of the erect 

 object from a double image of the object in- 

 verted, there is nothing at all mysterious to 

 any one who has learnt to consider how much 

 of the visual perception is referable to associa- 

 tion. If the light reflected from a single ob- 

 ject touched by us had produced, not two 

 merely, but two thousand, separate images in 

 our eyes, erect or inverted, or in any inter- 

 mediate degree of inclination, the visual feel- 

 ing thus excited would still have accompanied 

 the touch of a single object ; and if only it had 

 accompanied it uniformly, the single object 

 would have been suggested by it, precisely in 

 the same manner as it is now suggested by 

 the particular visual feeling that now attends 

 the double inverted image." * It has been 

 justly remarked by Dr. Alison, that if it were 

 only by experience and association with the 

 perceptions of touch that we learned that any 

 object placed before the eyes, and seen by 

 two images, is nevertheless single, we might 

 reasonably conclude that we should never see 

 an object double which we know by touch to 

 be single ; whereas we all know, that if by 

 pressure on the ball of one eye, or by any 

 other means we direct the axes of the two 

 eyes to different points in an object, we imme- 

 diately see it double, and cannot by any means 

 avoid seeing it double so long as that con- 

 dition of the eyes continues, notwithstanding 

 the full conviction, derived from touch, of its 

 being single. This tangible theory (if the ex- 

 pression may be used) has found iittle favour, 

 but having met with the support of so able a 

 man as Dr. Brown, it could not be passed over 

 in silence. 



Single Vision. When both eyes, acting 

 simultaneously, are directed to an object, a 

 single image only is seen. A variety of opinions 

 have existed with reference to this interesting 

 point, some of them sufficiently singular. 

 Gassendi, Du Tour, Porta, and Gall, for in- 

 stance, asserted that we do not make use of 

 more than one of our eyes at a time, the other 

 being relaxed, and inattentive to objects. Dr. 

 Brig^s supposed that single vision was owing 

 to the equal tension of the corresponding parts 

 of the optic nerves, whereby they vibrated in 

 a synchronous manner ; and Dr. Reid was of 

 opinion that the correspondence of the two 

 eyes, on which single vision depends, arose 

 from some natural constitution of the eye and 

 rnind. Porterfield sa}s that the true cause 

 why objects do not appear double, depends on 

 the' faculty we have of seeing things in the 

 place where they are, every point of an object 



Elements of Physiology, by Baly, p. 1171. 



Lecture 29. 



