OF OPTICS. 241 



But, that vision is effected in this manner, may be de- 

 monstrated experimentally. Take a bullock's eye whilst 

 it is fresh, and having cut off the three coats from the 

 back part, quite to the vitreous humour, put a piece of 

 white paper over that part, and hold the eye towards 

 any bright object, and you will see an inverted picture 

 of the object upon the paper. 



Seeing the image is inverted, many have wondered 

 why the object appears upright. But we are to con- 

 sider, 1. That inverted is only a relative term ; and 2. 

 That there is a very great difference between the real 

 object and the means or image by which we perceive it. 

 When all the parts of a distant prospect are painted 

 upon the retina, they are all right with respect to one 

 another, as well as the parts of the prospect itself; and 

 we can only judge of an object's being inverted, when it 

 is turned reverse to its natural position, with respect to 

 other objects which we see and compare it with. If we 

 lay hold of an upright stick in the dark, we can teA 

 which is the upper or lower part of it, by moving our 

 hand upward or downward ; and know very well that 

 we cannot feel the upper end by moving our hand down- 

 ward. Just so we find by experience, that upon direct- 

 ing our eyes towards a tall object, we cannot see its top 

 by turning our eyes downward, nor its foot by turning 

 our eyes upward ; but must trace the object the same 

 way by the eye to see it from head to foot, as we do by 

 the hand to feel it ; ami as the judgment is informed by 

 the motion of the hand in one case, so it is also by the 

 motion of the eye in the other. 



In the following diagram is exhibited the manner of 

 seeing the same object ABC, by both the eyes D and 

 E at once. 



When any part of the image c b a falls upon the 

 optic nerve L, the corresponding part of the object be- 

 comes invisible On which account nature has wisely 

 placed the optic nerve of each eye, not in the middle of 

 16. a 



