INVERSION OF IMAGES IN THE EYE. 47 



cross each other, as delineated in the figure, both before 

 and after passing into the eye. 



Yet we are not conscious that the image is inverted, but 

 see it as if it were erect, and the reason for this is that we 

 see the position and direction of objects only relatively to 

 what we see of our own bodies ; and as our bodies, and all 

 portions of our own bodies seen by us, have their images 

 inverted on the retina as well as the images of other 

 objects, the relative appearance and position of our own 

 bodies is in the image maintained by this inversion to the 

 relative appearance and position of other objects, so that 

 when we move our hand from one place to another, as 

 from the point to the feather of the arrow in the figure, 

 our hand would undoubtedly move downward, but its 

 inverted image on the retina would move upward ; so that 

 the hand would not in consequence afford us any means 

 by such a motion of discovering that the image was 

 inverted. In fact, every object external to the surface of 

 the eye, as must be apparent from a study of the 

 figure, is inverted, so that if the edge of the upper eyelid 

 were seen at as, its image would be presented along 

 the lower side of the retina at , and hence we have no 

 standard external to the eye itself by which we can detect 

 inversion as a matter of experience ; for every conceivable 

 standard external to the eye could only be presented to 

 our vision by means of its inverted image in the eye, and 

 hence before it could become visible to us it would have 

 suffered inversion itself as much as any object whose 

 inversion we meant to test by it. The explanation thus 

 given of the inversion of images, and our inability to 

 detect their inversion in our experience, is capable of being 

 proved even to strict mathematical demonstration. It 

 will thus be found on reflection, that neither the true sizes 

 nor the true positions of objects are presented to us by the 

 eye, but only their relative sizes and their relative positions. 



