CHAP, in.] SIGHT. 965 



from the apparent size, that is to say from the size of the retinal 

 image, only when the distance of the object from the eye is 

 known. Thus when an object gives rise to a retinal image of a 

 certain size, that is to say has a certain apparent size, we esti- 

 mate the distance from us of the object giving rise to the image, 

 and upon that come to a conclusion as to its real size. Con- 

 versely, when we see an object, of whose real size we are 

 otherwise aware, or are led to think we are aware, our judgment 

 of its distance is influenced by its apparent size. Thus when 

 part of our field of vision is occupied by the image of a man, 

 knowing otherwise the ordinary size of a man, we infer, if the 

 image be very small, that the man is far off. The reason of the 

 image being small may be because the man is far off, in which 

 case our judgment is correct; it may be, however, because the 

 image has been lessened by artificial dioptric means, as when 

 the man is looked at through an inverted telescope, in which 

 case our judgment becomes an illusion. So also a picture on a 

 magic lantern screen when gradually enlarged seems to come 

 forward, when gradually diminished seems to recede. In these 

 cases the influence which the absence of any muscular sense of 

 binocular adjustment or monocular accommodation ought to 

 bring to bear on our judgment, is thwarted by the more direct 

 influence of the association between size and distance. An 

 instructive illusion of a similar kind is produced by developing 

 in the eye a strong negative image ( 584) and projecting the 

 image on to a screen which is made to move backwards and 

 forwards, or is alternately inclined at various angles ; the nega- 

 tive image appears to change in size and shape, although it is 

 absolutely subjective in nature and wholly independent of the 

 movements of the screen. 



The complex reaction on each other of judgments as to dis- 

 tance and size is illustrated by the experience that an object such 

 as a person looks unnaturally large when seen in a fog ; being 

 seen indistinctly, he is judged to be farther off than he really is, 

 and so appears larger than he naturally would do at the distance 

 at which he is supposed to be ; and we are similarly influenced 

 by the greater or less brightness or saturation of colours. Con- 

 versely, distant mountains when seen distinctly in a clear atmos- 

 phere appear small, because on account of their distinctness they 

 are judged to be nearer than they really are. The indistinct- 

 ness of the image of the moon or sun when seen on the horizon, 

 similarly contributes to its appearing larger than when seen in 

 the zenith ; our judgment however is probably in this case also 

 due to our being better able to compare the moon or sun with 

 terrestrial objects. We seem moreover in this matter to be 

 especially influenced by our conception (which is itself an illus- 

 tration of the subject we have in hand) that the vault of the 

 heavens is flatter than it really is ; the zenith appears to be less 



