Kepler's explanation of objects appearing erect to us is, that the mind, perceiving the impulse of a ray on the lower part of the re tina, conceives this ray to be directed from a higher part of the object, and vice versa. Por terfield argues that the mind never sees any picture painted on the retina, and consequently never judges of the object from what it ob serves in the picture ; and that in seeing any 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 that erect vision results from the lines of 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 little favour, but having met with the support of so able a man as Dr. Brown, it could not be passed over in silence.
Single 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. Brigs 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 mind. Porterfield says 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 visible direction being in all cases perpen dicular to the impressed part of the retina ; but Miller offers the following " The hypothesis that erect vision is the re sult of our perceiving, not the image on the retina, but the direction of the rays of light which produce it, involves an impossibility, since each point of the image is not formed by rays having one determinate direction, bin 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.