Double and Inverted Images on the Retine. 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 retinz, are erect and single. 
This is done by reference to the Law of Visible Direction, fully 
illustrated by Dr Rerp, 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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