BINOCULAR VISION 691 



for colors, especially on the nasal side, as is seen in Fig. 1 77. The field 

 is smallest for green, a little larger for red, and is still larger for blue. 

 Investigation of the field of indirect vision with the perimeter is useful 

 in ophthalmology ; but the chief physiological interest, as regards the 

 sensibility of the retina, is connected with direct vision. 



BINOCULAR VISION 



Thus far the mechanism of the eye and its action as an optical 

 instrument have been described for monocular vision only ; but it is 

 evident that both eyes are habitually used, and that their axes are prac- 

 tically parallel in looking at distant objects and are converged when 

 objects are approached to the nearest point at which there is distinct 

 vision. In fact an image is formed simultaneously on the retina of either 

 eye, but it is nevertheless appreciated as a unit. If the axis of one eye 

 is slightly deviated by pressure on the globe, so that the images are not 

 formed upon corresponding points in the retina of either eye, vision is 

 more or less indistinct and is double. In strabismus, when this condition 

 is recent, temporary or periodical, as in recent cases of paralysis of the 

 external rectus muscle, when both eyes are normal, there is double 

 vision. When the strabismus is permanent and has existed for a long 

 time, double vision may not be observed, unless the subject directs the 

 attention strongly to this point. As but one eye is capable of fixing 

 objects accurately, images are formed on the fovea of this eye only. 

 Images formed upon the retina of the other eye are indistinct, and in 

 many instances are habitually disregarded ; so that practically the sub- 

 ject uses but one eye, and presents the errors of appreciation that attend 

 monocular vision, such as a want of exact estimation of the solidity and 

 distance of objects. It is stated as the rule that when strabismus of long 

 standing is remedied, so far as the axes of the eyes are concerned, by 

 an operation, binocular vision is not restored. This is explained on the 

 supposition that the perceptive power of the retina of the affected eye 

 has been gradually lost from disuse. In normal binocular vision the 

 images are formed on the fovea centralis of either eye ; that is, on cor- 

 responding points, which are, for each eye, the centres of distinct 

 vision. The concurrence of both eyes is necessary to the exact appre- 

 ciation of distance and form ; and when the two images are formed on 

 corresponding points, the visual centre receives a correct impression of 

 a single object. When vision is perfect, the sensation of the situation 

 of any single object is referred to one and the same point; and the 

 impression of a double image can not be received unless the conditions 

 of vision are abnormal. 



