80 4 SPECIAL SENSES. 



traversed the distance from one object to another. At first, all objects appeared to be, as 

 it were, painted upon a screen. Such points as these it would be impossible for us to. 

 accurately observe in infants ; but we have all seen young children grasp at remote 

 objects, apparently under the impression that they were within reach. It must be ad- 

 mitted,' however, that the case of Casper Hauser is rather indefinite; but it is certain 

 that, even in the adult, education and habit enable us to greatly improve the faculty of 

 estimating distances. 



The important questions for us now to determine relate to the differences between 

 monocular and binocular vision in the adult. We may see an object distinctly with 

 one eye ; but are we able, from an image made upon one retina, to appreciate all its 

 dimensions and its exact locality ? 



Accurate observations bearing upon this question leave no doubt of the fact that 

 monocular vision is incomplete and inaccurate, and that it is only when two images are 

 formed, one upon each retina, that vision is perfect. We cannot better illustrate the 

 truth of this proposition and the exact condition of our positive knowledge upon this 

 important point, than by quoting in full the facts and arguments advanced by Giraud- 

 Teulon : 



" Monocular vision only indicates to us immediately visual direction, and not precise 

 locality. At whatever distance a luminous point may be situated in the line of direction, 

 it forms its image upon the same point in the retina. 



" In the physiological action of a single eye, in order to arrive at an idea of the dis- 

 tance of a point in a definite direction, we have only the following elements : 



" 1. The consciousness of an effort of accommodation. 



" 2. Our own movement in its relations to the point observed. 



" 3. Facts brought to bear from recollection, education, our acquired knowledge with 

 regard to the form and size of objects : in a word, experience. 



" 4. The geometric perspective of form and position. 



" 5. Aerial perspective. 



" All these are elements wanting in precision and leaving the problem without a 

 decisive solution. 



" And, indeed : 



"We place before one of our eyes, the other being closed, the excavated mould of a 

 medallion : we do not hesitate, after a few seconds, to mistake it for the relief of the 

 medallion. This illusion ceases at the instant that both eyes are opened. 



" Or again : 



" A miniature, a photograph, a picture, produces for a single eye a perfect illusion ; 

 but, if both eyes be open, the picture becomes flat, the prominences and the depressions 

 are effaced. 



" We may repeat the following experiment described by Malebranche : ' Suspend by 

 a thread a ring, the opening of which is not directed toward us ; step back two or three 

 paces ; take in the hand a stick curved at the end ; then, closing one eye with the hand^ 

 endeavor to insert the curved end of the stick within the ring, and we shall be surprised 

 at being unable to do in a hundred trials what we should believe to be very easy. If, 

 indeed, we abandon the stick and endeavor to pass one of the fingers through the ring, 

 we shall experience a certain amount of difficulty, although it is very near. This diffi- 

 culty ceases at the instant that both eyes are opened.' 



As regards precision, exactitude of information concerning the relative distance 

 of objects, that is to say, the idea of the third dimension or of depth, there is then a 

 notable difference between binocular vision and that which is obtained by means of one 

 eye alone." 



It is evident that n accurate idea of the distance of near objects cannot be obtained 

 except by the use of both eyes, and this fact will explain, in part, the errors of monocu- 

 lar vision, when we look with one eye upon objects in relief ; for, under these conditions, 



