612 HANDBOOK OF PHYSIOLOGY. 



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 b in the image 

 on the retina increases as the angle becomes larger. Objects at different 

 distances from the eye, but having the same optical angles 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 conception of a visual space extend- 

 ing 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. 



Estimation of Size. Our estimation of the size of various objects 

 is based partly on the visual angle under which they are seen, but much 

 more on the estimate we form of their distance. Thus a lofty mountain 

 many miles off may be seen under the same visual angle as a small hill 

 near at hand, but we infer that the former is much the larger object be- 

 cause we know it is much further off than the hill. Our estimate of dis- 

 tance is often erroneous, and consequently the estimate of size also 

 Thus persons seen walking on the top of a small hill against a clear twi- 

 light sky appear unusually large, because we over-estimate their distance, 

 and for similar reasons most objects in a fog appear immensely magni- 

 fied. The same mental process gives rise to an idea of depth in the field 

 of vision, this idea being fixed in our mind principally by the circum- 

 stance that, as we ourselves move forwards, different images in succes- 

 sion become depicted in our retina, so that we seem to pass between these 

 images, which to the mind is the same thing as passing between the ob- 

 jects 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 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 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 remains constantly the same. But the imagination, 

 which analyzes the sensations of vision, invests the images of objects, to- 

 gether with the whole field of vision in the retina, with very varying 

 dimensions; the relative size of the image in proportion to the whole 

 field of vision, or the affected parts of the retina to the whole retina, 

 alone remaining unaltered. 



