276 



SENSE OF SIGHT. 



Fig. 124. 



placed at one end of a room, and direct the axes of both eyes to a cir- 

 cular aperture in a window-shutter at the other end, although an image 

 of this aperture may be formed in each eye, yet because the lines of 

 visible direction from similar points of the one image meet the lines 

 of visible direction from similar points of the other, each pair of 

 similar points will appear as one point, and the aperture seen by one 

 eye will exactly coincide with the aperture seen by the other. But if, 

 when an object is seen single with both eyes, we press one eye aside, 

 the image formed by that eye will separate from the other image, and 

 the object will appear double; or, if the axes of both eyes be directed 

 to a point either nearer or more remote than the aperture in the 

 window-shutter, then, in both these cases, the aperture will appear 

 double, because the similar lines of visible direction no longer meet at 

 the aperture. 1 In Fig. 122, if we look at the object A, the more dis- 

 tant object, B, will be seen double; and in Fig. 123, if we look at the 

 object B, the nearer object A will be seen double. It is not necessary, 



however, that the axes of the eyes should 

 be directed accurately on an object, in 

 order that it shall be seen single with both 

 eyes. A whole range of objects may be 

 seen single if their images are thrown on 

 corresponding parts of the retina in both 

 eyes, as in Fig. 124. 



After all, perhaps the true condition of 

 single vision is, that the two images of an 

 object should be formed on portions of the 

 two retinae that are accustomed to act in 

 concert. In cases of convergent strabis- 

 mus, the patient does not see double; but 

 immediately after a successful operation, 

 if the vision of the two eyes be good, he 

 does so; and this continues until the parts of the two retinae have 

 become habituated to act in concert. 



In the course of the preceding remarks, it was stated, that the eyes 

 are not always of the same power. The difference is sometimes sur- 

 prising. M. Adelon 2 mentions the case of a person, one of whose eyes 

 required a convex glass, with a focus of five inches; the other a concave 

 glass, with a focus of four inches. In these cases, it is important to 

 use one unassisted eye only ; as confusion must necessarily arise from 

 directing both to an object. This is the cause why we close one eye 

 in looking through a telescope. The instrument has the effect of ren- 

 dering the focal distance of the two eyes unequal, and of placing them 

 in the same situation as if they were, originally, of different powers. 



From what has been said it will be understood, that if from any 

 cause, as from a tumour pressing upon one eyeball, from morbid 

 debility of the muscles, or from want of correspondence in the sensi- 

 bility of the two retinae, the eyes be not properly directed to an object, 



1 Optics, p. 44, in Library of Useful Knowledge, Natural Philosophy, vol. i., Lond., 1829, 

 and Treatise on Optics, edit. cit. 

 3 Physiologie, edit, cit., i. 459. 



Binocular Vision. 



