n 4 o VISION. 



perception of the size of an object are the size of the retinal image and 

 the estimated distance of the object. The importance of the latter 

 factor is shown by an after-image experiment. If the after-image of 

 an object is projected on a moving screen, it will be seen to change 

 in size, becoming smaller as the screen approaches the eyes, larger as it 

 recedes. The retinal image upon which the after-image depends remains 

 constant in size, and the changes in apparent magnitude depend on the 

 projection. Emmert 1 measured the after-image at different distances, 

 and found the linear size of the image equal to the linear size of the 

 object, multiplied by the distance at which the image was seen. He 

 found that the changes were the same when accommodation was 

 paralysed, and occurred equally well in monocular and binocular vision. 

 The estimated distance of the after-image is therefore probably depend- 

 ent on psychological factors. It is known that judgments of size 

 are greatly influenced by these factors ; the large size of objects seen 

 indistinctly in a fog is a familiar example of the influence of aerial 

 perspective. 



Another instance which has excited much discussion is the apparently 

 greater size of the sun and moon at the horizon than at the zenith. 

 This is usually brought into relation with the apparent flattening of 

 the sky; the horizon appearing farther away than the zenith, the 

 moon appears larger at the former. A favourite explanation of the 

 larger size at the horizon is that in that position it can be compared 

 with intervening objects ; while, according to another, the appear- 

 ance is an instance of the illusion that a space filled with objects 

 appears longer than an unfilled space (see p. 1141). Another explana- 

 tion for which there is much to be said refers the larger size at the 

 horizon to greater indistinctness, consequently greater estimated distance 

 and greater apparent size. Helmholtz 2 found that a reflected image of 

 the moon at the zenith, seen so that it appeared near the horizon, was 

 not enlarged. Scharwin and Novizki 3 have observed corresponding 

 changes in the size of an after-image, when projected at horizon and 

 zenith. It is probable that the apparently greater size at the horizon is 

 due to more than one factor, greater indistinctness being the most 

 important. 



Micropsia and macropsia. — Illusions of size appear in certain con- 

 ditions which throw some light on the process of perception. The meta- 

 morphopsia of retinitis is often accompanied, especially in the early stages, by 

 micropsia. Objects may appear reduced to as much as one-fifth their natural 

 size. This micropsia is referred to the influences of inflammatory exudation 

 separating the retinal elements, so that a group of elements which are 

 normally covered by the retinal image of a small object require a very much 

 larger object in order to be covered, and the latter consequently appears 

 diminished in size. Occasionally macropsia is observed, and this is referred to 

 crowding of retinal elements together ; it is, however, much rarer than 

 micropsia, and the increase in size is comparatively slight. 



Similar illusions of size may occur under numerous other conditions. An 

 appearance of diminished size of small objects (printed type) may be due to 

 indistinct vision, the outer part of the type being hazy, while the central part 

 is seen distinctly. A more important effect is due to irradiation, 4 and is 



1 Klin. Monatsbl. f. Augenh., 1881, Bd. xix. S. 443. 



2 "Handbuch d. physiol. Optik," 1867, S. 631. 



3 Ztschr.f. Psychol, u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1S96, Bd. xi. S. 408. 



4 Mind, London and Edinb., 1896, N.S., vol. v. p. 71. 



