THE SENSES 805 



points could be pricked through with a needle. But since 

 the actual centre of the retina does not correspond with the 

 fovea centralis (Fig. 286), 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 circle produced 

 by pressing the blunt end of a pencil or the finger-nail on a point of 

 the globe of one eye, is not doubled by pressure over the corres- 

 ponding 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 follow- 

 ing the movement of the other, there is double vision. When squinting 

 is produced by paralysis of the third (p. 720) or the sixth cranial nerve 

 (p. 722), it is accompanied by diplopia, until in course of time the mind 

 learns to disregard one of the images. Here it is obvious that the 

 two images of an object cannot fall on corresponding points. 



But too much weight must not be allowed to such evidence, for 

 it is also a fact that 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. 806) show 

 clearly that images falling on non-corresponding points 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 



* 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 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 sesn 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 the 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). 



