DOUBLE VISION. 457 



pressure be made upon the upper part of one* eyeball, and the lower 

 part of the other, two luminous circles are seen, one above, the other 

 below ; in the same manner, pressure upon the inner sides of the two 

 eyes, produces two circles, and so also does pressure upon the outer 

 sides. The upper part of one retina, and the lower part of the other, 

 their inner sides and their outer sides, are said therefore not to be 

 identical. Speaking generally, those parts of the two retinae are iden- 

 tical which correspond in situation, in reference to the centre of the 

 eye ; thus, the upper portions, the lower portions, the right sides and 

 the left sides, corresponding in this relative position, are identical in 

 sensation. Hence, it has been said that all points of the two retinae, 

 situated at equal distances from their centres, and lying in the same 

 direction, are identical in sensation, and, when simultaneously excited 

 to action, give rise to the perception of one object. That a general 

 identity of action, or function, of certain parts of the two retinae, 

 exists, is sufficiently obvious; but that it does not aiford the ultimate 

 explanation of the combination of the two images, is shown by the 

 fact that, in stereoscopic vision, as we shall presently explain, the two 

 perspective views of a given solid object, which are necessarily dis- 

 similar, or they would not yield a stereoscopic effect, cannot cover 

 exactly identical parts of the two retinae, and yet they are combined 

 into one image or impression. Again, images formed on corresponding 

 points of the two retinae, are sometimes not combined, but are seen 

 double in the visual field, as when we look at one of two objects 

 placed exactly in front of us, between the eyes, the images of the one 

 which is seen double falling nevertheless on exactly identical points of 

 the two retinae. The same is the case, when we look at one object 

 with both eyes, but through different colored pieces of glass. 



Diagram K. 



It has been shown by Helmholz, that, in double vision, it is not ac- 

 tually vertical meridian lines on the retinae which correspond, or are 

 identical, but other lines inclined about 1J from the meridian, which 

 he calls apparently vertical meridian lines. The horizontal correspond- 

 ing lines are, on the contrary, actually horizontal. If, for example, 

 as in Diagram K, A, across a horizontal line, another be drawn, accu- 

 rately perpendicular to it, the right upper included angle appears 

 larger than a right angle to the right eye, and less than a right angle 

 to the left eye; the lower angles are altered in the reverse manner. 

 Again, if two figures be drawn, having similar horizontal lines, but 

 crossed by vertical lines inclined at their upper ends, 1J left and 

 right, from a central perpendicular, then, when the left-hand figure 

 (Diagram K, B) is looked at with the left eye only, the angles formed 



