THE SENSES. 



229 



Diplopia. Single vision results only when certain parts of the two 

 retinae are affected simultaneously; if different parts of the retinae receive 

 the image of the object, it is seen double. This may be readily illus- 

 trated as follows: The eyes are fixed upon some near object, and one of 

 them is pressed by the thumb so as to be turned slightly in or out; two 

 images of the object (Diplopia or Double Vision) are at once perceived, 

 just as is frequently the case in persons who squint. This diplopia is due 

 to the fact that the images of the object do not fall on corresponding 

 points in the two retinae. 



The parts of the retinae in the two eyes which thus correspond to each 

 other in the property of referring the images which affect them simulta- 

 neously to the same spot in the field of vision, are, in man, just those 

 parts which would correspond to each other, if one retina were placed 



FIG. 386. 



FIG. 387. 



exactly in front of, and over the other (as in Fig. 386, c). Thus the outer 

 lateral portion of one eye corresponds to, or, to use a better term, is iden- 

 tical with, the inner portion of the other eye; or a of the eye A (Fig. 386), 

 with a' of the eye B. The upper part of one retina is also identical with 

 the upper part of the other; and the lower parts of the two eyes are iden- 

 tical with each other. 



This is proved by a simple experiment. Pressure upon any part of 

 the ball of the eye, so as to affect the retina, produces a luminous circle, 

 seen at the opposite side of the field of vision to that on which the pressure 

 is made. If, now, in a dark room, we press with the finger at the upper 

 part of one eye, and at the lower part of the other, two luminous circles 

 are seen, one above the other: so, also, two figures are seen when pres- 

 sure is made simultaneously on the two outer or the two inner sides 

 of both eyes. It is certain, therefore, that neither the upper part of one 

 retina and the lower part of the other are identical, nor the outer lateral 

 parts of the two retinas, nor their inner lateral portions. But if pressure 



