BINOCULAR VISION. 715 



" 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 distance 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, experi- 

 ence. 



" 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 degree of difficulty, although it is very near. 

 This difficulty 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 an accurate idea of the distance of near objects can 

 not be obtained except by the use of both eyes., and this fact will partly ex- 

 plain the errors of monocular vision in looking with one eye upon objects in 

 relief ; for under these conditions, it is impossible to determine with accu- 

 racy 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 is obtained by the use of both eyes ; for the estimation of dis- 

 tance 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, 



