738 HANDBOOK OF PHYSIOLOGY. 



an opposite or antagonistic condition of that membrane ; and the opposite 

 conditions of which the retina thus becomes the subject would seem to 

 balance each other by their reciprocal reaction. A necessary condition 

 for the production of the contrasted colors is, that the part of the retina 

 in which the new color is to be excited, shall be in a state of comparative 

 repose ; hence the small object itself must be gray. A second condition 

 is, that the color of the surrounding surface shall be very bright, that is, 

 shall contain much white light. 



BINOCULAR VISION. 



Although the sense of sight is exercised by the two eyes, yet the im- 

 pression of an object conveyed to the mind is single. Various theories 

 have been advanced to account for this phenomenon. 



By Gall it was supposed that we do not really employ both eyes si- 

 multaneously in vision, but always see with only one at a time. This 

 especial employment of one eye in vision certainly occurs in persons 

 whose eyes are of very unequal focal distance, but in the majority of 

 individuals both eyes are simultaneously in action, in the perception of 

 the same object; this is shown by the double images seen under certain 

 conditions. 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 

 illustrated 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) 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 

 simultaneously to the sanu spo; in the field of vision, are, in man, just 

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

 placed exactly in front of, and over the other (as in fig. 436). Thus, 

 as we have noticed in speaking of the distribution of the optic nerve- 

 fibres, the temporal portion of one eye corresponds to, or, to use a better 

 term, is identical with the nasal portion of the other eye; or a of the 



