LECT. XX. SINGLE VISION. 379 



The intensity of the light which we receive from an 

 object, and which we know diminishes with the distance, 

 assists us also in judging of the distance; but such a judg- 

 ment is rendered uncertain by atmospheric variations, which 

 continually modify the quantity of light received by the 

 object. Lastly, in forming an opinion respecting the real 

 magnitude of objects which are more or less distant from us, 

 we depend partly on our estimate of their distance, and 

 partly on their apparent magnitude, which is measured by 

 the size of the images produced on the retina. The errors 

 that we frequently commit when estimating distance, are 

 one source of the illusions in judging of real magnitude: 

 these are most frequently made in the dark, as in the phan- 

 tasmagoria. 



Single Vision. What is the use of the two eyes in the 

 act of vision ? Whilst the object is situated very far from 

 us, the images formed in the two eyes are identical, and 

 vision is effected as with one eye, the optical axes being 

 then sensibly parallel. The single impression produced by 

 a body seen with two eyes is, in this case, the result of an 

 intellectual act, which, from custom, we execute with an 

 inconceivable rapidity. We do not perceive two objects, 

 although the image be double, because experience has 

 taught us that this object is single in every case where two 

 identical representations are produced upon two parts of 

 the retina, which are necessarily correspondent, in order 

 that vision may be distinct. It is the same with the organ 

 of touch. If we place all the fingers of one hand on a ball, 

 we do not experience the sensation of five balls, but only 

 of one. If we look at a body with two eyes, and then 

 compress the globe of one of them so as to change the posi- 

 tion of the image on the retina, and alter the axis of one of 

 the two eyes, the body instantly appears double. This is 



