2.54< optics. 



But that vision is effected in this manner maybe 

 demonstrated experimentally. Take a bullock's 

 eye while 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. 



Since the image is inverted, many have won- 

 dered why the object appears upright. But we are 

 to consider, 1. That inverted is only a relative 

 term; and, °Z. 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 tell 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 downward. Just so we find by 

 experience, that upon directing 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 ; and 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. 



The diameters of images at the bottom, of the 

 eye are proportional to the angles which the ob- 

 jects subtend at the eye, the same as in a lens; 



IS 



