VISUAL DIRECTION. 515 



ceived by the mind to have an extent of two miles perhaps, if 

 we know that its extent is such, or if we infer it to be so from 

 the number of known objects seen at the same time. And in 

 the same way that the images of several different objects, 

 viewed under the same angle, thus appears to the mind to have 

 a different size in the field of vision, so the whole field of vision 

 which has always the same absolute size, is interpreted by the 

 mind as of extremely various extent ; and for this reason 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 circumstance that, 

 as we ourselves move forward, different images in succession 

 become depicted on our retina, so that we seem to pass be- 

 tween these images, which to the mind is the same thing as 

 passing between the objects themselves. 



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 pres- 

 ent to it ; and our own body, with which they come into con- 

 tact, is the measure of their size. The part of a table touched 

 by the hand appears as large as the part of the hand receiv- 

 ing 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 re- 

 mains constantly the same. But the imagination, which ana- 

 lyzes 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 propor- 

 tion 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 retina are projected 

 into the exterior world, the relation of the images to each 

 other remains the same. 



The estimation of the form of bodies by sight is the . result 

 partly of the mere sensation, and partly of the association of 

 ideas. Since the form of the images perceived by the retina 



