REVERSION OF IMAGE ON RETINA. 513 



eye. Volkrnann has also shown that a similar experiment may 

 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 account 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. Miiller and Volkmann are of opinion 

 that the mind really perceives an object as inverted, but needs 

 no correction, since everything 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 

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

 thing as the daily inversion of objects consequent on the revo- 

 lution of the entire earth, which we know only by observing 

 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 regu- 

 lating 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, there- 

 fore, to be the result of an act of the mind. And this leads us 

 to a consideration of the several other properties of the retina, 

 and of the co-operation of the mind in the several other parts 

 of the act of vision. To these belong not merely the act of 

 sensation itself, and the perception of the changes produced 

 in the retina, as light and colors, but also the conversion of 

 the mere images depicted in the retina into ideas of an ex- 

 tended field of vision, of proximity and distance, of the form 

 and. size of objects, of the reciprocal influence of different 

 parts of the retina upon each other, the simultaneous action of 

 the two eyes, and some other phenomena. 



To speak first of the ideal size of the field of vision : The 

 actual size of the field of vision depends on the extent of the 

 retina, for only so many images can be seen at any one time 



