220 HAND-BOOK OF PHYSIOLOGY. 



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 1 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 extendsd 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 at the sa^me time; and thus considered, the 

 retina, of which the affections are perceived by the mind, 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 projecting 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 impedi- 

 ments near the eye: on the contrary, it is very extensive when the pro- 

 jection 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 extensive 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 extent 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. By a mental 

 operation of this kind we obtain a correct idea of the size of individual 

 objects, as well as of the extent of the field of vision. To illustrate this, 

 it will be well to refer to Fig. 382. 



The angle x, included between the decussating central rays of two 

 cones of Tight issuing from different points of an object, is called the 

 optical angle angulus options sen visorius. This angle becomes larger, 



