THE SENSES. 611 



tion 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 conver- 

 sion 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 phe- 

 nomena. 



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 anyone 

 time as can occupy the retina at 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 projecting images 

 on the retina towards the exterior. Hence the mental field of vision is 

 very small when the sphere of the action of the mind is limited to im- 



Fia. 412. Diagram of the optical angle. 



pediments near the eye: on the contrary, it is very extensive when the 

 projection of the images on the retina towards the exterior, by the in- 

 fluence of the mind, is not impeded. It is very small when we look in- 

 to 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 

 Tiew 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. 412. 



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

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

 optical angle angulus opticus seu visorius. This angle becomes larger, 



