234 PSYCHOLOGY. 



of the riglit angles in the same figure makes it seem larger 

 than the other. In Fig. 63, the retinal image of the space 

 between the extreme dots is in all three lines the same, yet 

 it seems much larger the moment it is filled up with other 

 dots. 



In the stereoscope certain pairs of lines which look 

 single under ordinary circumstances immediately seem 

 double when we add certain other lines to them.* 



Ambiguous Import of Eye-movements. 



These facts show the indeterminateness of the space- 

 import of various retinal impressions. Take now the eye's 

 movements, and we find a similar vacillation. When we 

 follow a moving object with our gaze, the motion is ' volun- 

 tary ' ; when our eyes oscillate to and fro after we have 

 made ourselves dizzy by spinning around, it is ' reflex ' ; 

 and when the eyeball is pushed with the finger, it is ' pas- 

 sive.' Now, in all three of these cases we get a feeling 

 from the movement as it efi'ects itself. But the objective 

 perceptions to wL icli the feeling assists us are by no means 

 the same. In the first case we may see a stationary field 

 of view with one moving object in it ; in the second, the 

 total field swimming more or less steadily in one direction ; 

 in the third, a sudden jump or twist of the same total 

 field. 



The feelings of convergence of the ej-eballs permit of the 

 same ambiguous inter j)retation. When objects are near we 

 converge strongly upon them in order to see them ; when 

 far, we set our optic axes parallel. But the exact degree of 

 convergence fails to be felt ; or rather, being felt, fails to 

 tell us the absolute distance of the object we are regarding. 

 Wheatstone arranged his stereoscope in such a way that the 

 size of the retinal images might change wdthout the con- 

 vergence altering ; or conversely, the convergence might 

 change without the retinal image altering. Under these 

 circumstances, he says,t the object seemed to approach or 

 recede in the first case, without altering its size , in the 

 second, to change its size without altering its distance — just 



*A. W. Volkmann, Uutersuchungen, p. 253. 

 f Philosophical Transactious, 1852, p. 4. 



