694 SPECIAL SENSES 



" 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 : 



" i. 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, ex- 

 perience. 



11 4. The geometric perspective of form and position. 



"5. Aerial perspective. 



" All these are elements wanting in precision and leaving the prob- 

 lem without a decisive solution. 



" And, indeed : 



" We place before one of our eyes, the other being closed, the exca- 

 vated 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 are 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 dimen- 

 sion, 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 explain the errors of monocular vision in looking with 

 one eye at objects in relief ; for under these conditions it is impossible to 

 determine with accuracy whether the points in relief are nearer or farther 

 from the eye than the plane surface. This will not fully explain, how- 

 ever, the idea of solidity of objects, which is obtained by the use of both 



