730 HANDBOOK OF PHYSIOLOGY. 



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

 the retina, are equally inverted, and therefore maintain the same rela- 

 tive 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 posi- 

 tion. 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 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 extended 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. 



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 as can occupy the retina to the same time ; and thus considered, 

 the retina, the conditions of which are perceived by the brain, is itself 

 the field of vision. But to the mind of the individual the size of the 

 field of vision has no determinate limits; sometimes it appears very 

 small, at another time very large ; for the mind has the power of pro- 

 jecting images on the retina toward the exterior. Hence the mental 

 field of vision is very small when the sphere of the action of the mind is 

 limited to impediments near the eye: on the contrary, it is very exten- 

 sive when the projection of the images on the retina toward the exterior, 

 by the influence of the mind, is not impeded. It is very small when 

 we look into a hollow body of small capacity held before the eyes ; large 

 when we look out upon the landscape through a small opening ; more ex- 

 tensive when we look at the landscape through a window ; and most so 

 when our view is not confined by any near object. In all these cases the 

 idea which we receive of the size of the field of vision is very different, 

 although its absolute size is in all the same, being dependent on the ex- 

 tent of the retina. Hence it follows, that the mind is constantly co- 

 operating in the acts of vision, so that at last it becomes difficult to say 

 what belongs to mere sensation, and what to the influence of the mind. 



