\ in OCULAR MOVEMENTS 407 



paralytic squint diplopia is tin 1 most disturbing symptom to the 

 patient; in concomitant xyuint, on the contrary, the patients do 

 not generally enmplaiii of diplopia, either bemuse they are 

 accustomed to ne^lt ct the image in the squinting eye 3 <>r because 

 the disparate points at which the images in this eye are formed 

 ha\e acquired the property of the I'ovca of the other eye, so that 

 the two images are superposed in consciousness. 



VI. Another interesting phenomenon of binocular vision is 

 that known as the struggle between the visual fields of the two 

 eyes. The simplest case in which this occurs is when we look 

 at a white surface through lenses of differently coloured glass; 

 it is difficult to see the hue resulting from the physical mixture of 

 the two colours; usually there is a successive perception of one or 

 the other colour over the whole Held, or one colour prevails in one 

 part of the field and the other colour in the other part, according 

 as the sensation in one or the other eye prevails in all or certain 

 parts of the two retinae. If the observation is prolonged the 

 retinal sensibility to colour becomes blunted, chromatic perception 

 is less unstable and variable, but also more indefinite and dimmed, 

 so that it approximates to grey. 



These effects are even better seen when, instead of lenses with 

 different coloured glasses, two differently coloured fields are in- 

 spected in a mirror or prism stereoscope. 



It is a moot point whether during the alternating predominance 

 of one or the other colour-sensation there is any true binocular 

 mixture of the two colours. Dove, Briicke, Ludwig, Panum, and 

 Heriug claim to have observed this mixture ; on looking, for 

 instance, with one eye at yellow and with the other at blue, they 

 occasionally saw blue-green. The author has observed the same 

 effect. H. Meyer, Volkmann, Funke, and Helmholtz, on the 

 contrary, never succeeded in obtaining a true binocular colour- 

 mixture. This is probably due to individual idiosyncrasy, and 

 perhaps as Hering suggests different observers may interpret 

 the mixture of the two colours with which they are experimenting 

 differently. 



If instead of two fields of different colours two discs, one white 

 and the other black, are viewed binocularly, a sensation is obtained 

 of shimmering grey, now darker and now lighter, with a peculiar 

 lustre, which cannot be detected on gazing at either of the discs 

 alone with one or both eyes. The brightness of objects depends 

 --ntially on the fact that no point of a shining surface can appear 

 equally bright to both eyes, because it does not reflect the same 

 ;i mount of light in all the different, directions. Silk, for instance, 

 shines because each thread reflects the light differently in different 

 directions and the mass of its constituent threads does not present 

 a smooth and homogeneous surface. The lustre in binocular 

 vision of black and white depends on the fact that the elementary 



