SENSE OF VISION PERCEPTION OF SOLIDITY. 763 



even supposing the fact to be as stated, would be far from affording the solu- 

 tion of the problem ; and it would be entirely inapplicable to that very im- 

 portant series of phenomena to be next described, which show how large an 

 amount of information we derive, not from the repetition, but from the differ- 

 ence, of the sensory impressions made by the same object upon our two retinae; 

 and which indicate that here, as in the case of erect vision, the mental inter- 

 pretation of the sensory impressions is a process altogether removed from the 

 simple affection of the consciousness by those impressions, and is not to be 

 accounted for by any structural arrangements of the Sensorial apparatus. 

 One condition of single Vision, however, seems to be this, that the two 

 images of the object should be formed on parts of the two retime which are 

 accustomed to act in concert ; and habit appears to be the chief means by 

 which this conformity is produced. There can be no doubt, however, that 

 double images are continually being conveyed to the Sensorium ; but that, 

 from their want of force and distinctness, and from the attention being fixed 

 on something else, we do not take cognizance of them. This may be shown 

 by a very simple experiment. If two fingers be held up before the eyes, one 

 in front of the other, and vision be directed to the more distant, so that it is 

 seen singly, the nearer will appear double ; while, if the nearer one be re- 

 garded more particularly, so as to appear single, the more distant will be 

 seen double. A little consideration will show, therefore, that our minds 

 must be thus continually affected with sensations, which cannot be united 

 into the idea of a single image; since, whenever we direct the axes of our 

 eyes towards any object, almost everything else will be represented to us as 

 double ; but we do not ordinarily perceive this, from our minds being fixed 

 upon a clear and distinct image, and disregarding, therefore, the vague un- 

 defined images formed by objects not in the visual focus. Of this it is very 

 easy to satisfy one's self. It has been found by Gehler, J. Mu'ller, and 

 others, that those objects which do not appear double when a given point is 

 fixedly regarded are situated in a certain circular line, termed the Horopter, 

 of which the chord is formed by the distance between the eyes, whilst the 

 magnitude of the circle is determined by three points, namely, by the two 

 eyes and the point towards which their axes converge. Prof. Helmholtz, 

 however, considers that generally the Horopter is a line of double curvature, 

 produced by the intersection of two hyperboloids, or exceptionally, of two 

 plane curves ; and that when we look straight forward to a point of the 

 horizon, the Horopter becomes a horizontal plane passing through our feet 

 being, in fact, the ground on which we are standing. The form and situa- 

 tion of the Horopter appear to be of great practical importance in enabling 

 us to judge with accuracy of the shape and distance of those objects which 

 are situated in it. 1 The above experiment, moreover, makes it evident that 



nerve, it was found that the Optic Tract upon the same side as the lesion, showed no 

 signs of degeneration, whilst that of the opposite side soon underwent disorganiza- 

 tion with the exception of a few strands on the inner and posterior aspect; thus 

 countenancing the opinion that there are commissural fibres passing between the cor- 

 pora quadrigemina of the two sides, but negativing the view that some of the fibres 

 from each retina run to the corpora quadrigemina on their own side. Dr. Waller is 

 inclined to think, therefore, that the decussation is complete. It is important to 

 observe, that in these experiments the anterior portion of the divided nerve, with the 

 retinal elements, remained unchanged for many weeks. See Proceedings of the 

 Royal Society, vol. viii. On the other hand, the reader may refer to some curious 

 cases of partial Amaurosis recorded by Mr. Towne in the Guy's Hospital Reports for 

 1863, p. 127. 



1 See Miiller's Physiology, translated by Dr. Baly, vol. ii, p. 1196; also the OOOH- 

 ian Lecture for 1864, delivered before the Royal Society, by Professor Helmholtz, in 

 vol xiii of the Proceedings of the Royal Society, and Dr. H. Kaiser, in Archiv f. 

 Ujihth., Bd. xv, 1869, p. 75. Dr. Kaiser, who has treated the subject analytically, 



