588 PHYSIOLOGY 



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 investigation 

 of the corresponding points by means of the haploscope. In this instrument a white 

 screen is placed vertically at the far point of vision of the eyes, which are made some- 

 what 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 lin:s 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. 



It might be thought that single vision with the two eyes was due to the 

 fact that the nerve fibres from corresponding points in the two retinae pass to 

 and terminate in identical nervous structures in the brain. This is not the 

 case. Single vision is largely the result of experience, and we can show by 

 experiment that both elements are present in the fused sensation obtained 

 from the two eyes. Thus if the left eye is directed on to a red surface and 

 the right eye on to a blue surface, we may obtain either a fusion of the fields 

 with the production of a purple colour, or a struggle of the two fields so that the 

 whole field appears sometimes red and sometimes blue. In the same way if 

 two figures, lying side by side, one ruled with vertical lines and the other with 

 horizontal lines, be looked at so that the image of the right figure falls on the 

 right retina and of the left figure on the left retina, we do not see a single 

 figure with the lines crossing one another so as to form a network, but we 

 again obtain a struggle so that first one figure is seen with the vertical lines 

 prominent, and then this wanes and is succeeded by the other figure in which 

 the horizontal lines are prominent. Which figure we see most easily seems 

 to be dependent on the direction of the eye movement. If the eyes are 

 moved vertically upwards and downwards the figure with the vertical lines 

 comes more into prominence, while the figure with the horizontal lines is seen 

 when the eyes are moved from side to side. 



