124 SPECIAL SENSES. 



ceding experiment ; but it is necessary in addition to know 

 the angle formed by the other side with the base, or the, 

 length of one of the sides, which cannot be known exactly 

 except by opening the other eye. The arrangement of the 

 two eyes as regards the angle formed by the visual rays (read 

 the lines of vision) which meet each other at the object is 

 then one of the best and most universal means made use of 

 by the intelligence to judge of the distance of things? '' 



From the above extract, which is entirely in accordance 

 with our present knowledge, it is evident that an accurate 

 idea of the distance of objects cannot be obtained except by 

 the use of both eyes, and this fact will explain, in part, the 

 errors of monocular vision, when we look with one eye upon 

 objects in relief ; for, under these conditions, w r e cannot de- 

 termine with accuracy whether the points in relief be nearer 

 or farther from the eye than the plane surface. This will 

 not fully explain, however, the idea of solidity of objects 

 which we obtain by the use of both eyes ; for the estimation 

 of distance is obtained by bringing the axes of both eyes to 

 bear upon a single object, be it near or remote. The fact is, 

 as was distinctly stated by Galen, in the second century, 1 that, 

 when we look at any solid object not so far removed as to 

 render the visual axes practically parallel, we see with the 

 right eye a portion of the surface which is not seen with the 

 left eye, and vice versa. The two impressions, therefore, are 

 not identical for each retina ; the image upon the left retina 

 including a portion of the left side of the object not seen by 

 the right eye, the right image in the same way including a 

 portion of the right surface not seen by the left eye. These 

 slightly dissimilar impressions are fused, as it were, produce 

 the impression of a single image, when vision is perfectly 

 normal, and this gives the idea of relief or solidity, enabling 

 us to appreciate exactly the form of objects, when they are 

 not too remote. 



1 GALIEN, (Euvres anatomiques, physiologiques et medicales, par DAREMBERG, 

 Paris, 1854, tome i., p. 640, et seq. 



