514 THE SENSE OF SIGHT. 



as can occupy the retina, at the same time ; and thus consid- 

 ered, 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 the 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 impediments near the eye : on the contrary, it is 

 very extensive when the projection of the images on the retina 

 towards the exterior, by the influence of the mind, is not im- 

 peded. 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 understand this, it will be necessary to 

 refer again to Fig. 185, p. 512. 



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 sen visorim. This 

 angle becomes larger, the greater the distance between the 

 points A and B ; and since the angles x and y are equal, the 

 distance between the points a and 6 in the image on the retina 

 increases as the angle x becomes larger. Objects at different 

 distances from the eye, but having the same optical angle, x 

 for example, the objects, c, d, and e, must also throw 

 images of equal size upon the retina ; and if they occupy the 

 same angle of the field of vision, their image must occupy the 

 same spot in the retina. 



Nevertheless, these images appear to the mind to be of very 

 unequal size when the ideas of distance and proximity come 

 into play ; for from the image a b, the mind forms the concep- 

 tion of a visual space extending to e, d, or c, and of an object 

 of the size which that represented by the image on the retina 

 appears to have when viewed close to the eye, or under the 

 most usual circumstances. A landscape depicted on the 

 retina, as a b, and viewed under the angle x, is therefore con- 



