LECT. XX. CASE OF ERECT VISION. 377 



How do we appreciate the position, the distance, and the 

 size, in a word, the mode of existence of an object, and 

 its relations to surrounding bodies? What is ihe office of 

 the two eyes ? 



What we have hitherto said, has been for the purpose 

 of proving that the image of an object is formed upon the 

 retina, and that it is distinct, but is inverted, as regards the 

 object itself; and that this double effect is produced what- 

 ever may be the distance between the object and the eye. 

 Nevertheless, this image is not yet the sensation ; this only 

 takes place when the modification experienced by the reti- 

 na, has been transmitted to the seat of perception by means 

 of the optic nerve. 



Cause of erect Vision. But how does vision result from 

 this modification made upon the retina by the rays which 

 external objects transmit there ? The first question that 

 presents itself to our notice, without occupying ourselves 

 with the metaphysical part of the subject, is, that of the 

 position of objects. It has been repeatedly said, by way 

 of explaining the inversion of the images as regards the 

 objects they represent, that we see the produced images 

 in an inverted position. To see objects in the inverted 

 position of their images, is what we call seeing objects 

 erect. In appreciating the position of bodies and their 

 erection, we merely compare the position of their different 

 parts with that of surrounding bodies. Without this, the 

 words reversed and erected as applied to objects, would be 

 devoid of all meaning. To us, a man is in the upright 

 position when his feet are towards the earth, and his head 

 in the part most distant from it; and the reversed image 

 which he forms upon the retina, in no way deranges the 

 respective position of the various parts of the man in re- 

 rard to the earth. In the image, the feet are equally nearer 



