Double and Inverted linages on the Retince. 477 



is, that the impression on the upper part is from the lower 

 object, and that on the lower part from the higher. Why this 

 difference should exist, is the point in question. 



It is perhaps difficult to find decisive evidence in the human 

 body, that the intimation of the position of the erect object, 

 given by the inverted image, is originally correct, and in harmo- 

 ny with the intimation given through the sense of touch, before 

 experience and association can have time to operate ; but it is un- 

 necessary to argue this point, because it is allowed even by Dr 

 Brown, that, in the case of many of the lower animals, there is 

 an original perception of the true position of objects, acquired 

 by the sense of sight ; so that those who have so humble an idea 

 of their own powers of visual perception as to believe, that it is 

 only by experience and association that they learn to judge, by 

 the eye, whether an object is erect or inverted, may acquiesce 

 in what is here to be said, as applicable to the lower animals. 



But it is farther necessary to premise here, that, while some 

 philosophers have thought it unnecessary to seek for any expla- 

 nation of correct vision by double and inverted images, because 

 they believe that the eye gives no original intimations whatever 

 as to the number or position of objects, — others are equally con- 

 vinced of the futility of the inquiry, because they maintain, 

 that the eye does give intimations which necessarily imply, that 

 the objects, of which inverted images are formed on correspond- 

 ing points of the two retinae, are erect and single. 



This is done by reference to the Law of Visible Direction, fully 

 illustrated by Dr Reid, and many others, according to which, 

 every object appears to be in the direction of a straight line 

 drawn perpendicularly to the retina at the point where its image 

 is formed. This law has not been similarly expressed by all who 

 have referred to it ; but the terms here used are those employed 

 by Sir D. Brewster and Mr Mayo. In conformity with this 

 law, when an image is formed on a concave surface, the lower 



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