924 A MANUAL OP PHYStOWCY 



fovea, every point of the one retina will be covered by the corre- 

 sponding point of the other retina, so that identical points could be 

 pricked through with a needle. But since the actual centre of the 

 retina does not correspond with the fovea centralis (Fig. 381), 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. 819) or the sixth 

 cranial nerve (p. 822), 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 

 example, 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 stereoscopic vision (p. 925) 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 multitudes of objects are constantly falling on points of 

 the retinae not anatomically ' 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 inter- 

 section of the visual lines, or the part to which the gaze is directed 



* 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 hor- 

 opter is practically 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). 



