18 SIR DAVID BREWSTER ON HEMIOPSY, OR HALF-VISION. 



them. If hemiopsy is produced by the distended blood-vessels of the retina, 

 these vessels must be similarly distributed in each eye, and similarly affected by 

 any change in the system ; and, consequently, must produce the same effect upon 

 each retina, and upon the same part of it. 



In explaining single vision with two eyes, we have no occasion to appeal to 

 double fibres in the optic nerves, or to corresponding points on the retina. There 

 is, in reality, no such thing as single vision, that is a single image seen by both 

 eyes. With two sound eyes every object is seen double, and it appears single 

 only when, by the law of visible position, the one image is placed above the other. 

 But even in this case the object is seen double, by means of two dissimilar images 

 of it which are not coincident. By shutting the right e}'e, we lose sight of a part 

 on the right side of the double image, which is seen only by the right eye ; and 

 by shutting the left eye, we lose sight of a part on the left side of the double 

 image, which is seen only by the left eye. If one eye gives a better picture than 

 the other, the duplicity of the apparently single image is more easily seen. By 

 shutting the good eye the imperfect picture is seen, and by shutting the bad eye 

 we insulate the perfect picture. It is difficult to understand how optical writers 

 and physiologists should have so long demanded a single sensation for the pro- 

 duction of a single picture from the two pictures imprinted on the two retinas. 

 If we had the hundred eyes of Argus, the production of an apparently single 

 picture would have been the necessary result of the Law of Visible Position. 



