

CAUSE OF ERECT VISION. 257 



from the lower part on the upper; hence the image of the object is 

 reversed, as in Fig. 113. It has, accordingly, been asked; how it is, 

 that we see the object in its proper position, as its image is inverted on 

 the retina ? Buffon, 1 Le Cat, 2 and others believed, that, originally, 

 we do see them so inverted ; but that the sense, of touch apprises us of 

 the error, and enables us to correct it at so early a period, and so 

 effectually, that we are afterwards not aware of the process. This 

 cannot apply, however, to the lower animals; and, accordingly, the 

 knot has been cut by the supposition and there is much to favour it 

 that in them it is innate or intuitive. 3 Berkeley, 4 again, asserted, 

 that the position of objects is always judged of, by comparing them 

 with our own ; and that, as we see ourselves inverted, and this view is 

 embraced by Muller, Volkmann, 5 and numerous others, external bodies 

 are in the same relation to us as if they were erect. It is not necessary 

 to reply at length to these views. Cases enough have occurred of the 

 blind from birth having been restored to sight to show, that no such 

 inversion, as that described by Buifon, takes place ; and the boy, who 

 stoops down, and looks at objects between his legs, although he may 

 be, at first, a little confused, from the usual position of the images on 

 the retina being reversed, soon sees as well in that way as in any other. 

 The great error with all these speculatists has been, that they have 

 imagined a true picture to be formed on the retina, which is regarded 

 by the mind, and therefore seen inverted. It need hardly be said, that 

 there is no interior eye to take cognizance of this image; but that the 

 mind accurately refers the impression, made upon the retina, to the 

 object producing it; and if the lower part of the retina be impressed 

 by a ray from the upper part of an object, this impression is conveyed 

 by the retina to the brain as it receives it, and no error can be indulged. 

 Professor Alison 6 offers an explanation, first suggested to him by Mr. 

 Dick, veterinary surgeon, which turns on the alleged fact, that the 

 course of the optic nerves and tractus optici is such, that impressions 

 on the upper part of the retina are, in fact, impressions on the lower 

 part of the optic lobes, that impressions on the outer part of the former 

 are on the inner part of the latter, and conversely. 



When a cone of light proceeds from a radiant point, as from B, Fig. 

 113, the whole of the rays, whatever maybe their relative obliquity, 

 are, as has been seen, converged to a focus upon the retina at &, yet 

 the point B is seen only in one direction, in that of the central ray or 

 axis of the cone B b. If we look over the top of a card at the point B, 

 till the edge of the card is just about to hide it ; or if, in other words, 

 we obstruct all the rays that pass through the pupil, excepting the 

 uppermost, the point is still seen in the same direction as when it was 

 viewed by the whole cone proceeding from B. If we look, again, be- 

 neath the card, in a similar manner, so as to see the object by the low- 



i Memoires de 1' Academic, 1743, p. 231. 2 Op. citat. 



3 Carpenter, Human Physiology, p. 266, Lond., 1842. 



4 Essay on Vision, 2d edit., p. 60, Dublin, 1709. 



5 Wagner's Handworterbuch der Physiologic, 14te Lieferung, s. 342, Braunschweig, 1846. 



6 On Single and Correct Vision, by means of Double and Inverted Images on the Retinae, 

 in Transact, of the Royal Society of Edinburgh, vol. xiii., Edinb., 1836. 



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