4H On the Causes of Erect and Si?igle Vision. 



version whatever: for there is no relative variety by comparison 

 with any other set of similar images; and they will necessarily 

 bear a given relative proportion to the ideas of motion and tan- 

 gibility; and which ideas, taken collectively, include all the ele- 

 ments we have of the knowledge of the position, figure, and co- 

 lour of objects*" 



No doubt the relations of these in indefinite modifications 

 are perceived by the judgement, as well as innumerable associa- 

 tions of them by the imagination ; and thence the large use of 

 vision in the world ; thence the warm affections which are ap- 

 proved of by the understanding, or delighted in by the fancy. 



But instead of taking this simple and easy mode of viewing 

 the subject, philosophers, when they discuss the reason of 

 erect vision, really suppose (although they may not be willing 

 to allow it in so many words), that mental vision arises from, 

 and is occupied about, two sets of objects at the same time ; 

 viz. the external objects in nature, and the inverted images of 

 them on the retina: whereas the external object becomes vir- 

 tually null and void immediately upon the rays of light being 

 emitted from it. 



The idea of inversion is the result of the comparison of the line 

 of demarcation of one object with that of another of a similar 

 kind placed in a contrary direction to it. But as in the picture 

 on the retina, the line of demarcation of each particular image 

 touches the line of demarcation of the rest, in the same man- 

 ner and after the same proportion as their corresponding ob- 

 jects do in external nature ; so no such comparison can take 

 place : for one set of images only is painted, and these in pre- 

 cisely the same relative positions to each other as are their 

 counterparts. The mind therefore necessarily perceives the 

 same positions with respect to each other; for no two objects 

 of a kind present themselves, by which a comparison can take 

 place. 



Philosophers, therefore, when they compare the image on 

 the eye of an ox, for instance, with the object in external na- 

 ture of which such image is the reflection, forget that both to- 

 gether make but one picture on their own eyes: For any 

 given object forms on the human eye an inverted image, and 

 the mind sees it erect; but the image on the eye of the ox 

 (which is already inverted) makes on the eye of the person who 

 observes it, an image again inverted that is erect, and the 

 mind perceives it inverted. 



In this latter case there is a comparison of the line ofdemar- 



* See u Essays on the Perception of an External Universe," &c. by 

 Lady Mary Shepherd. 182/. Essay, xiv. p. 408. 



cation 



