THE EYE AND THE PHYSIOLOGY OF VISION. 581 



visual sensation. Every normal individual when looking 

 directly at an object sees It as one object. It is possible, 

 however, to see objects double, of the truth of which one 

 may easily satisfy himself by holding a pencil before his 

 eyes and then looking past the pencil into the distance. 

 He will perceive two pencils. Of course if he turns his 

 focus upon the pencil before him the two pencils will melt 

 into one and the object appear single. But in that case 

 the objects in the distance appear more or less double. 

 There are on the retinas corresponding points, the simul- 

 taneous stimulation of which produces in the mind but a 

 ^single sensation, and we ordinarily see objects single be- 

 cause we turn to the object, corresponding points. If, on 

 the other hand, the image of anything should fall on points 

 which do not correspond, each image will produce its own 

 sensation and so we see double. 



To determine which are corresponding points of the ret- 

 ina it is only necessary to imagine the retina of one eye 

 laid right over the retina of the other eye in such a way 

 that the yellow spots would coincide as well as the merid- 

 ians passing through the yellow spots. In that case the 

 points lying immediately above and below each other are 

 corresponding points. For a point in the upper and outer 

 part of the retina of the right eye, the corresponding point 

 of the left eye would be found in the upper inner portion. 

 Reference to Figures 181 and 182 will help to clarify this. 

 Thus, in Figure 181 the gaze is directed for the point/", 

 which is seen as a single point because its images at c and 

 c fall on corresponding points, but the point g is seen 

 double at G and G 2 because the images of it fall on points 

 g and g 2 which do not correspond. The corresponding 

 point to g is of course g' A . Turning now to the next fig- 

 ure in which the point G is seen single because its images 

 at c and c fall on corresponding points, f is seen double at 

 F' and F~ , because the images of point f fall at f and/"', 

 which are not corresponding points, the corresponding 

 point to f being/" 3 . 



