654 THE SENSE OF SIGHT. 



be successfully performed in a living person possessed 

 of large prominent eyes, and an unusually transparent 

 sclerotica. 



No completely satisfactory explanation has yet been 

 offered to acccount for the mind being able to form a 

 correct idea of the erect position of an object of which an 

 inverted image is formed on the retina. Muller and 

 Volkmann are of opinion that the mind really perceives 

 an object as inverted, but needs no correction, since every- 

 thing is seen alike inverted, and the relative position of 

 the objects therefore remains unchanged : and the only 

 proof we can possibly have of the inversion is by experi- 

 ment and the study of the laws of optics. It is the same 

 thing as the daily inversion of objects consequent on the 

 revolution of the entire earth, which we know only by ob- 

 serving the position of the stars ; and yet it is certain that, 

 within twenty-four hours, that which was below in relation 

 to the stars, comes to be above. Hence it is, also, that no 

 discordance arises between the sensations of inverted vision 

 and those of touch, which perceives everything in its erect 

 position ; for the images of all objects, even of our own 

 limbs, in the retina, are equally inverted, and therefore 

 maintain the same relative position. Even the image of 

 our hand, while used in touch, is seen inverted. The 

 position in which we see objects, we call therefore the 

 erect position. A mere lateral inversion of our body in a 

 mirror, where the right hand occupies the left of the 

 image, is indeed scarcely remarked : and there is but little 

 discordance between the sensations acquired by touch in 

 regulating our movements by the image in the mirror, and 

 those of sight, as, for example, in tying a knot in the 

 cravat. There is some want of harmony here, on account 

 of the inversion being only lateral, and not complete in all 

 directions. 



The perception of the erect position of objects appears, 

 therefore, to be the result of an act of the mind. And this 



