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HANDBOOK OF PHYSIOLOGY. 



tions. If two fingers be held up before the eyes, one in front of the 

 other, and vision be directed to the more distant, so that it is seen 

 singly, the nearer will appear double; while, if the nearer one be regarded, 

 the most distant will be seen double; and one of the double images in 

 each case will be found to belong to one eye, the other to the other eye. 

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

 retinae are affected simultaneously; if different parts of the retinae re- 



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

 lustrated 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; 



FIG. 418. 



two images of the object (Diplopia or Double Vision) are at once per- 

 ceived, just as is frequently the case in persons who squint. This diplo- 

 pia is due to the fact that the images of the object do not fall on corre- 

 sponding 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 si- 

 multaneously 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 



