656 TH E SEXSE OF SIGHT. 



also, the image viewed in the camera obscura is regarded 

 as a real landscape as the true field of vision although 

 only a small image depicted upon paper. The same mental 

 process gives rise to the idea of depth in the field of vision; 

 this idea being fixed in our mind principally by the cir- 

 cumstance that, as we ourselves move forwards, different 

 images in succession become depicted on our retina, so that 

 we seem to pass between these images, which to the mind 

 is the same thing as passing between the obje-cts them- 

 selves. 



The action of the sense of vision in relation to external 

 objects is, therefore, quite different from that of the sense 

 of touch. The objects of the latter sense are immediately 

 present to it ; and our own body, with which they come 

 into contact, is the measure of their size. The part of a 

 table touched by the hand appears as large as the part of 

 the hand receiving an impression from it, for a part of our 

 body in which a sensation is excited is here the measure 

 by which we judge of the magnitude of the object. In the 

 sense of vision, on the contrary, the images of objects are 

 mere fractions of the objects themselves realized upon the 

 retina, the extent of which remains constantly the same. 

 But the imagination, which analyzes the sensations of 

 vision, invests the images of objects, together with the 

 whole field of vision in the retina, with very varying 

 dimensions ; the relative size of the images in pro- 

 portion to the whole field of vision, or of the affected 

 parts of the retina to the whole retina, alone remaining 

 unaltered. 



The direction in which an object is seen, the direction of 

 vision, or visual direction, depends on the part of the retina 

 which receives the image, and on the distance of this part 

 from, and its relation to, the central point of the retina. 

 Thus, objects of which the images fall upon the same parts 

 of the retina lie in the same visual direction ; and when, 

 by the action of the mind, the images or affections of the 



