vin OCULAR MOVEMENTS 409 



of one or the other image over the whole or certain parts of the 

 binocular Held is, according to H. Meyer, Helmholtz, and Fechner, 

 due to oscillations of the attention, which is automatically con- 

 centrated on the content of first one and then the other of the 

 visual fields. Panum, on the contrary, gives a physiological 

 explanation of the phenomenon, and refers it to the conflict of 

 outlines. In his opinion the contours or limits between white 

 and black excite the retina more forcibly than the evenly 

 illuminated surfaces, and produce an irregular rhythm of 

 excitation in the cerebral centres. 



A further series of effects, studied particularly by Fechner 

 (1860), and more in detail later on by Brlicke, Meyer, Panum, 

 and Helmholtz, are known as the phenomena of binocular contrast. 

 On closing one eye, and looking at a coloured surface for a few 

 seconds with the other, and then gazing with both eyes at a black 

 card with a vertical white band down the middle, so that two 

 half-images of this band result (physiological diplopia), it is found 

 that the half -image of the fatigued eye shows the complementary 

 colour of the inducing tint (negative secondary half -image), 

 while the half-image of the resting eye shows the contrast- colour 

 (though possibly faded), that is, the same colour that has acted on 

 the other eye (positive secondary half -image). If, for example, 

 the right eye is fatigued with red, the half-image of the white 

 band seen with this eye appears greenish on a black ground, 

 while the half-image of the band seen with the left eye is pink. 

 This difference between the half-images is not due to the fact 

 that they are formed at non-corresponding points of the retinae, 

 because Helmholtz noted the same effect of binocular contrast, 

 when the white band on the black ground is fixated so that its 

 image falls on the fovea of the rested or of the fatigued eye, i.e. 

 on identical points of the two retinae. 



Fechner attempted to explain the phenomena of binocular 

 contrast as the after-effects of the excitation of both retinae; 

 Helmholtz, on the other hand, again invokes the intervention 

 of psychical factors which have no definite physiological basis, 

 that is, he regards them as mere illusions or errors of judgment. 

 We must be content to regard the explanation of binocular 

 contrast as doubtful, like that of the so-called " theory of cerebral 

 visual images " (p. 371). 



VII. Corporeal or spatial vision that is, perception of the 

 three dimensions of external objects is possible with using only 

 one eye. But the solidity of uniocular visual images is not so 

 much an immediate, simple sensation as a true perception or 

 visual judgment, based on light and shade, and on move- 

 ments of the fixation - point or the head, with corresponding 

 changes in the projection and perspective of the images. When 

 light and dark shading, and the changes in perspective due to 



