656 PHYSIOLOGY 



lying to one side of us. Such an action is, however, associated with con- 

 siderable effort, and in nearly all cases is replaced by movements of the 

 head. Whenever we wish to examine an object closely we turn the 

 head so that the object lies between the two eyes, and a simple move- 

 ment of convergence serves to bring the image of the object on to the 

 two foveae centrales. 



BINOCULAR VISION CORRESPONDING POINTS 

 When we fix our gaze on any object, although an image of the 

 object is formed on each retina, the object appears as single and not 

 as double. On the other hand, gentle pressure on one eyeball, so as to 

 shift its gaze slightly from that imposed on it by the co-ordinated action 

 of the ocular muscles, at once causes the object to appear double. This 

 shows that the single appearance of an object seen with the two eyes 

 is due to the fact that the image from this object must fall upon 

 points in the retina, simultaneous stimulation of which produces only 

 a single sensation. These points are known as ' corresponding 

 points' It is evident that to each point in one retina only one point 

 can exist in the other retina which corresponds to it, since for every 

 position of the eyes a luminous point can only throw an image on a 

 definite point in each retina. 



Such corresponding points are, in the first place, the central spots in 

 each retina. When we look at any spot the axes of the above eyes are 

 so directed that the image of the spot falls upon the fovea centralis 

 of the two retinae. The image of all points to the right of this spot will 

 fall on the nasal side of the right retina and on the temporal side of the 

 left retina, and vice versa. If the right retina were cut out and placed 

 on the left, the corresponding points of the two retinae would be nearly 

 over one another. 



This statement is not absolutely accurate, as is shown by a careful investiga- 

 tion of the corresponding points by means of the haploscope. In this instru- 

 ment a white screen is placed vertically at the far point of vision of the eyes, 

 which are made somewhat myopic by means of a convex lens. Each eye looks 

 through a cylindrical tube, the axis of which coincides with the visual axes. 

 On each half of the white field is a mark which, however, appears single, 

 since its image falls on the fovea centralis of the two retinae. If in the left 

 field of vision a line be drawn vertically upwards from the mark, and in the 

 right field of vision another line vertically downwards, the two lines appear 

 as a single line passing through the mark. It will be seen that the upper 

 half of the line is not absolutely continuous with the lower half, but appears to 

 form a slight angle with it, showing that the general statement made above is 

 not absolutely correct. By means of this instrument we can determine the 

 extent and situation of all the points in the two retinae which correspond. 

 The totality of all the points in space, the lines from which to the eyes will fall 

 on corresponding points, is known as the horopter. Its determination is, 

 however, only of theoretical interest. 



