568 Dr. Readers Experiments for [April, 



We next come to the rectification of inverted images on the 

 retina. This, according to Scheiner snd Kepler, is the business 

 of the mind, which, when it perceives an impression on the lower 

 part of the retina, considers it as made by rays proceeding from 

 the higher parts of the object tracing the rays back to the pupil 

 where they cross one another. But this hypothesis (saj^s Dr. 

 Priestley, a great metaphysician) will hardly he deemed satisfac- 

 tory ; and, by way of clearing up the difficulty, he proceeds 

 thus : " llpper and lower are only relative terms ; and as all 

 objects are painted upon the retina in a similar manner (all the 

 tipper parts in one direction, and all the lower parts in the other), 

 it is by custom onlv, founded on experience and the association 

 of ideas, that we learn to distinguish them from one another, so aa 

 to direct our eyes, or point our hands upwards or downwards, as 

 we have occasion. If this be the true solution (continues the 

 learned Doctor) it will follow that if the images of objects had 

 always been painted in a different manner, that is, erect as the 

 objects themselves are, we should have acted as we do now 

 without being sensible of the difference, a different association 

 of ideas only having taken place." Now all this laboured expla- 

 nation comes to nothing more or less than that we are taught by 

 experience. However, we never find the infant or the brute 

 (incapable of these refined associations) mistaking the top for 

 the botttom, or the right for the left. When the world was 

 turned upside down by philosophers, they should have attributed 

 the circumstance to blind instinct, and not to reason. For indeed 

 reason has nothing whatever to do with the business. In the 

 summer of 1812, 1 performed the operation for cataract on a very 

 intelhuent boy, named Edward Carey, aged 10 years ; he was 

 born with such opaque cataracts 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. 



Althou'ih Cheseldon was an advocate for the retina being the 

 seat of-.vision, he does not make any particular observations on 

 this diff.culty, but says 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 what 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 mistake 

 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 of right and left, 



