998 THE SENSES 



But since the actual centre of the retina does not correspond with the 

 fovea centralis (Fig. 397), but lies nearer the nasal side, the nasal edge 

 of the left retina will overlap the temporal edge of the right, and the 

 nasal edge of the right will overlap the temporal edge of the left; so 

 that a part of each retina has no corresponding points in the other. 



The adherents of this theory claim, and with justice, that a small 

 object so situated that its image must be formed on corresponding 

 points of the two retinae does, as a rule, appear single, and, what is even 

 more striking, that a phosphene, or luminous ring produced by pressing 

 the blunt end of a pencil or the finger-nail on a point of the globe of one 

 eye (which Newton compared to the circles on a peacock's tail), is not 

 doubled by pressure over the corresponding point of the other eye, 

 although two circles are seen when pressure is made upon points which 

 do not correspond. If in rotating the eyes one eye is prevented by 

 pressure with the finger from following the movement of the other, 

 there is double vision. When strabismus or squinting is produced by 

 paralysis of the third (p. 895) or the sixth cranial nerve (p. 897), it is 

 accompanied by diplopia, until in course of time the mind learns to 

 disregard one of the images. In some cases of squint the double images 

 are never completely suppressed, but a new abnormal form of visual 

 localization is developed, which, however, very seldom permits any 

 accurate judgment of depth. In strabismus it is obvious that the two 

 images of an object cannot fall on corresponding points. 



But it is also a fact that, under certain conditions, images situated 

 on corresponding points may not, and that images not situated on 

 corresponding points may, give rise to a single impression. For ex- 

 ample, if one of the closed eyes be held slightly out of its ordinary 

 position by the finger, pressure on identical points of the two eyes gives 

 rise to two separate phosphenes. And some of the phenomena of stere- 

 oscopic vision (p. 999) show clearly that images falling on points not 

 strictly corresponding may give a single impression; while we do not 

 habitually see double, although it is certain that the images of multi- 

 tudes of objects are constantly falling on points of the retinae not ana- 

 tomically ' identical/* 



The question therefore arises, How is it that we do not see these 

 double images? This is one of the difficulties of the theory of identical 

 points. The following is a partial explanation: (i) The images of 

 objects in the portion of the field most distinctly seen that is, the 

 portion in the immediate neighbourhood of the intersection of the 

 visual lines, or the part to which the gaze is directed are formed on 

 identical points; and by rapid movements the eyes fix successively 

 different parts of the field of view. (2) Vision grows less distinct as 

 we pass out from the centre of the retina, and we are accustomed to 

 neglect the blurred peripheral images in comparison with those formed 



* In every fixed position of the eyes, the objects whose images fall on 

 corresponding points will be arranged on certain definite lines or surfaces 

 which vary with the direction of the visual axis and to which the name of 

 horopter, or point-horopter, has been given. For most eyes when directed 

 to the horizon that is, with the visual axes parallel the horopter is practi- 

 cally the horizontal plane of the ground, so that all objects within the field 

 of vision, and resting on the ground, fall upon corresponding points, and are 

 seen single. When the eyes are directed to a point at such a distance that 

 the lines of vision are sensibly convergent, the horopter consists (i) of a 

 straight line drawn through the fixing-point and at right angles to a plane 

 passing through the fixing-point and the two visual lines (visual plane) ; (2) of 

 a circle passing through the fixing-point and the nodal points of the two eyes 

 (the famous horopteric circle of Miiller). 



