1 12 2 VISION. 



of simple luminosity, having little to do with localisation ; and more 

 specialised visual elements, the basis of distinct perception. He 

 supposes that colour is due to the simultaneous activity of the two 

 kinds of retinal element ; to interference between the two sets of 

 vibrations started by their stimulation. He formerly believed visual 

 purple to be the basis of the photo-aesthetic activity, but since this 

 substance is absent from the fovea, and since the presence of only 

 one kind of element would involve the colour-blindness of this part 

 of the retina, he has lately 1 given up the idea that visual purple is 

 the source, or the only source, of the photo-sesthetic activity. The other 

 kind of vibration he supposes to be set up by thermo-electric action 

 between the cones and the retinal pigment. Charpentier thinks it 

 probable that the retinal oscillations described by him (p. 1074) depend 

 on transverse vibration of the free ends of the rods and cones. 



BINOCULAR VISION, MOVEMENTS OF THE EYES, AND 

 SPATIAL PERCEPTION. 



Introductory. — The point of especial interest in this part is the 

 physiological basis of visual perception of space. Its consideration 

 involves psychological factors to a much greater extent than the subject 

 of the second part of this article. The physiological factors are of two 

 kinds, namely, movements of the extrinsic and intrinsic ocular muscles 

 with the central motor processes connected with their innervation, and the 

 binocular mechanism by means of which the two eyes are brought into 

 physiological association with each other, and are made to act in many 

 ways as if they were one organ. Certain facts about these two sets of 

 factors must first be considered. 



Binocular combination. — In studying binocular vision, it is neces- 

 sary to have some means of presenting different objects to the two 

 eyes. This may be done without apparatus, by placing the object on 

 the lines of direction of the eyes, either beyond or nearer than the 

 fixation point, when the combined image is seen accompanied by the 

 lateral images. The observation of the combined image may be 

 assisted by screens to hide the lateral images. Some individuals, 

 especially myopes, obtain binocular combination most readily with the 

 lines of vision crossed beyond the objects, and with normal vision this 

 method is usually rendered more easy by wearing weak convex glasses. 

 Probably most individuals, however, obtain more satisfactory results by 

 fixing a point nearer than the objects to be combined. Of the different 

 forms of stereoscopic or haploscopic arrangement, which have been used 

 for the same purpose, the most suitable for physiological experiments is 

 Wheatstone's, especially in the modified form of Hering's haploscope. 



Retinal rivalry.— When the fields presented to the two eyes are 

 different, a phenomenon known as " retinal rivalry " occurs. If two 

 glasses differing in brightness are held, one before each eye, the 

 combined field may be observed to become periodically lighter and 

 darker ; with glasses of different colour, the field may change periodically 

 in colour. The greater the difference in brightness between the two 

 fields, the more marked is the rivalry. When there is in one field an 



1 Arch, d'opht., Paris, 1896, tome xvi. p. 337. 



