CHAP. IL] SIGHT. 539 



tressing, and form an important element in some kinds of delirium, 

 such as delirium tremens. 



Appreciation of apparent size. By the eye alone we can only 

 estimate the apparent size of an object, we can only tell what space 

 it takes in the field of vision, we can only perceive the dimensions 

 of the retinal image, and therefore have a right only to speak of 

 the angle which the diameter of the object subtends. The real 

 size of an object must be determined by other means. But our 

 perception of even the apparent size of an object is so modified by 

 concurrent circumstances that in many cases it cannot be relied 

 on. The apparent size of the moon must be the same to every 

 eye, and yet while some persons will be found ready to compare 

 the moon in mid heavens with a threepenny piece, others will 

 liken it to a cart-wheel ; that is to say, the angle subtended by the 

 moon seems to the one to be about equal to that subtended by a 

 threepenny piece held at the distance from the eye at which it is 

 most commonly looked at, and to the other about equal to that sub- 

 tended by a cart-wheel similarly viewed at the distance at which 

 it is most commonly looked at. If a line such as AC, Fig. 73, be 

 divided into two equal parts AB, BC, and AB be divided by 

 distinct marks into several parts, as is shewn in the figure, while 

 BG be left entire, the distance AB will always appear greater than 

 CB. So also, if two equal squares be marked, one with horizontal 

 and the other with vertical 

 alternate dark and light bands, 



the former will appear higher, 



and the latter broader, than it 



really is. Hence short persons affect dresses horizontally striped 

 in order to increase their apparent height, and very stout persons 

 avoid longitudinal stripes. Two perfectly parallel lines or bands, 

 each of which is crossed by slanting parallel short lines, will appear 

 not parallel, but diverging or converging according to the direction 

 of the cross-lines. 



Again, when a short person is placed side by side with a tall 

 person, the former appears shorter and the latter taller than each 

 really is. The moon on the horizon appears larger than when at 

 the zenith, because in the first position it can be most easily com- 

 pared with terrestrial objects. The absence of comparison may, 

 however, contribute to an opposite effect, as when a person looks 

 larger 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. So, conversely, 

 distant mountains when seen distinctly in a clear atmosphere 

 appear small, because on account of their distinctness they are 

 judged to be nearer than they really are. Indeed, our daily life is 

 full of instances in which our direct perception is modified by 

 circumstances. Among those circumstances previous experience is 



