122 SPECIAL SENSES. 



upon each retina, that vision is perfect. We cannot better 

 illustrate the truth of this proposition and the exact condi- 

 tion of our positive knowledge upon this important point, 

 than by quoting in full the facts and arguments advanced by 

 Giraud-Teulon : l 



" 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 di- 

 rection, we have only the following elements : 



" 1. The consciousness of an effort of accommodation. 



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

 served. 



" 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 medal- 

 lion. 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 



1 GIRAUD-TEULON, La vision binoculaire. Revue des cours stientifiques, Paris, 

 1867-1868, tome v., p. 225, et seq. 



