THE SENSE OF VISION. 359 



separated parts of the nervous system. Yet but a single object is perceived. 

 The phenomenon is the same as that which occurs when a grain of sand is 

 held between the thumb and ringer. In both eases we have learned (chiefly 

 through the agency of muscular movements and the nerves of muscular sense) 

 to interpret the double sensation as produced by a single object. 



Any two points, lying one in each retina, the stimulation of which by rays 

 of light gives rise to the sensation of light proceeding from a single object are 

 said to be " corresponding points." Now, it is evident that thefovexe centrales 

 of the two eyes must be corresponding points, for an object always appears 

 single when both eyes are fixed upon it. That double vision results when the 

 images are formed on points which are not corresponding may be best illus- 

 trated by looking at three pins stuck in a straight rod at distances of 35, 45, 

 and 55 centimeters from the end. If the end of the rod is held against the 

 nose and the eyes directed to each of the three pins in succession, it will be 

 found that, while the pin looked at appears single, each of the others appears 

 double, and that the three pins therefore look like five. 



The two foveoB centrales are not, of course, the only corresponding points. 

 In fact, it may be said that the two retinas correspond to each other, point for 

 point, almost as if they were superposed one upon the other with the foveas 

 together. The exact position of the points in space which are projected on to 

 corresponding points of the two retinas varies with the position of the eyes. 

 The line or surface in which such points lie is known as the " horopter." A 

 full discussion of the horopter would be out of place in this connection, but 

 one interesting result of its study may be pointed out — viz. the demonstration 

 that when, standing upright, we direct our eyes to the horizon the horopter is 

 approximately a plane coinciding with the ground on which we stand. It is 

 of course important for security in walking that all objects on the ground 

 should appear single, and, as they are known by experience to be single, the 

 eye has apparently learned to see them so. 



Since the vertical meridians of the two eyes represent approximately rows 

 of corresponding points, it is evident that when two lines are so situated that 

 their images are formed each upon a vertical meridian of one of the eyes, the 

 impression of a single vertical line will be produced, for such a line seen bin- 

 ocularly LS the most frequent cause of this sort of retinal stimulation. This 

 is the explanation commonly given of the singular optical illusion which is 

 produced when lines drawn as in Figure 17b' are looked at with both eyes fixed 

 upon the point of intersection of the lines and with the plane in which the 

 visual axes lie forming an tingle of about 20° with thai of the paper, the dis- 

 tance of the lines from the eyes being such thai cadi line will lie approximately 

 in the same vertical plane with one of the visual axes. Under these circum- 

 stances each line will form its image on a vertical meridian of one of the eves. 

 and the combination of these images results in the perceptiou of a third line, 

 not lying in the plane of the paper, but apparently passing through ii more or 

 less vertically, and swinging round its middle point with every movement of 

 the head or the paper. In this experiment it will be found thai the illusion 



