THE PERCEPTION OF SIGHT. 279 



to ascertain whicli signs go together, and which correspond 

 in different regions of the retina to points at similar 

 distances in the two dimensions of the field of vision. 

 This is in accordance with experiments by Fechner,^ 

 Volkmann,'^ and myself, which prove that even the fully 

 developed eye of an adult can only accurately compare 

 the size of those lines or angles in the field of vision, the 

 images of which can be thrown one after another upon 

 precisely the same spot of the retina by means of the 

 ordinary movements of the eye. 



Moreover, we may convince ourselves by a simple ex- 

 periment that the harmonious results of the perceptions 

 of feeling and of sight depend, even in the adult, upon a 

 constant comparison of the two, by means of the retinal 

 pictures of our hands as they move. If we put on a pair 

 of spectacles with prismatic glasses, the two flat surfaces 

 of which converge towards the right, all objects appear to 

 be moved over to the right. If we now try to touch any- 

 thing we see, taking care to shut the eyes before the hand 

 appears in sight, it passes to the right of the object ; but 

 if we follow the movement of the hand with the eye, we 

 are able to touch what we intend, by bringing the retinal 

 image of the hand up to that of the object. Again, if 

 we handle the object for one or two minutes, watching 

 it all the time, a fresh correspondence is formed between 

 the eye and the hand, in spite of the deceptive glass, 

 so that we are now able to touch the object with per- 

 fect certainty, even when the eyes are shut. And we 

 can even do the same with the other hand without seeing 

 it, which proves that it is not the perception of touch 



' Gustav Theodor Fechner, author of Elemente der Psychophysik, 1860 ; 

 also known as a satirist. — Tb. 



"^ Alfred Wilhelm Volkmann, s\iccessive1y Professor of Physiology at . 

 Leipzig. Dorpat, and Halle; author oi Physiologische JJnters'uchungen im 

 Gehiete der Ojptik, 1864, &c.— Tb. 



