Experiments on a new Theory of Vision. 57 



capable of these refined associations) mistaking the top for the 

 bottom, or the right for the left. When the world was turned 

 upside down bv philosophers, they should have attributed the cir- 

 cumstance to blind instinct, and not to reason : indeed, reason has 

 nothing whatsoever to do with the business. In the summer of 

 1812 I performed the operation for cataract on a very intelligent 

 boy, named Edward Casay, aged ten years. He was born with such 

 opaque cataracts (according to his mother's account) as merely 

 to enable him to distinguish light from darkness, or the shadow 

 of an interposed hand, but was incapable of distinguishing the 

 outlines of any object or the most brilliant colours. After the 

 operation, and before he could acquire any ideas from association, 

 having inquired the manner in which he saw ; he answered that 

 he saw objects as he felt them, supposing them to be very near 

 the eyes. Although Cheselden was an advocate for the retina 

 being the seat of vision, he does not make any particular observa- 

 tions on this difficulty ; but savs that the young gentleman whom 

 he couched with congenital cataracts " knew not the shape of 

 any object, nor any thing from another, however different in shape 

 or magnitude ; but upon being told wliat things were, whose form 

 he before knew from feeling, he would carefully observe that he 

 might know them again." When shown his father's picture, and 

 told what it was, he acknowledged a likeness, but did not mis- 

 take the head for the feet. Indeed, were there no other difficulty 

 in the retinal theory of vision, the inversion of objects, or turning 

 the world upside down, and making confusion right and left, 

 would be sutficient to invalidate the entire. The next difficulty 

 in this catalogue of difficulties is the power of seeing objects di- 

 stinctly at different distances. It is allowed on all ha'.ids, that to 

 see an object at different distances, either the retina or crystalline 

 lens must approach, so as to shorten what is called the optic axis. 

 So that the crystalline, according to the retinal theory, must be 

 a great and incessant jumper. To calculate the number of jumps 

 or little leaps the lens of a general officer would take at a review, 

 might puzzle the most able algebraist ; vulgar arithmetic would 

 be inade(|uatc to the solution ; and then an able philosopher has 

 given thousands and tens of thousands of muscles (or wings if 

 you please to call them so) to this little busy fluttering thing. In- 

 deed, the justly celebrated Dr. Young might as well have given 

 muscles to an onion, the lamcllie of which and those of the 

 crystalline are very similar. On examining the different theories 

 on this subject, we find them all to differ, and jjcrfcctly inadequate 

 to the effect. Kepler two centuries ai;o sui)posed that the con- 

 tracting of the ciliary processes draws the sides of the eye towards 

 the crystalline; by which means the eye is lengthened, and the re- 

 tina 



