1058 VISION. 



mentary to the original. This anomalous coloration is usually due to the 

 abnormal nature of the white light used, or to the influence of the light 

 passing through the sclerotic and iris. After-images are very delicate 

 means of observing slight differences of colour, partly owing to their 

 relatively low degree of brightness, which permits slight differences of 

 colour to be discriminated, partly to the absence of differences of marking 

 and texture, which in many surfaces mask slight differences of colour. 

 It is usually possible to get rid of anomalous colour by excluding lateral 

 light. In some cases, however, it is possible that unusual coloration may 

 be due to an abnormal condition of the retina (Hilbert). 1 



If a fixed surface is gradually replaced by a different surf ace, the change 

 which has been taking place during fixation may gradually pass into the 

 after-image. If the change is sudden, there is in the ordinary method 

 of observation an interval between the disappearance of the surface and 

 the appearance of the negative or complementary image. This interval 

 is partly filled up by phenomena to be described later, requiring special 

 means of observation. If the eyes are kept steadily fixed, the after- 

 image may be observed to undergo periodical variations. The negative 

 (or complementary) image disappears and reappears, the intervals being 

 sometimes observed to be occupied by a positive (or homochromatic) image. 

 Plateau, 2 who first described the appearance, observed nine variations, 

 and a still larger number may be seen. The earlier variations occur at 

 intervals of about three to four seconds, the later variations more rapidly, 

 and the latter resemble in many respects the variations of minimal 

 sensations which have been usually ascribed to variations of attention. 

 These periodical variations of after-images were referred by Helmholtz 3 

 to movements of the eyes. They do not, however, accompany the 

 ordinary blinking movements of the eyelids, and the regularity of their 

 occurrence is against this supposition. After-images are, however, 

 influenced by movements of the eyes. Any sudden movement, or 

 sudden change of accommodation, will make an after-image disappear ; 

 but during gradual changes after-images may be kept steadily in 

 view. 



The shortest time of exposure necessary to produce an after-image 

 effect is very small. A coloured patch on a rotating disc will give rise, 

 at certain rates, and with bright illumination, to a ring in the comple- 

 mentary colour of the patch. The after-image effect is strong enough to 

 more than neutralise the effect of the original excitation, and shows that 

 the complementary image occurs with exposure of a small fraction of a 

 second. 



According to Exner, 4 there are three parts of the spectrum which are much 

 less altered in appearance by fatigue than other parts, namely, red from the 

 end of the spectrum to between the lines C and D, green between E and b, 

 and blue about G. On the other hand, v. Kries 5 found yellow, green, and blue 

 least changed in colour-tone, red and yellow-green changing their hue in the 

 direction of yellow, blue-green and violet in the direction of blue. Hess 6 

 found that the appearance of a spectral light after fatigue corresponded in 

 general to that of a mixture of this light with the complementary of the 



1 Ztschr.f. PsychoL u. Physiol. d. Sinnesorg., Hamburg u. Leipzig, 1893, Bd. iv. S. 74. 



2 Ann. d. Phys. u. Chem., Leipzig, 1834, Bd. xxxii. S. 543. 



3 "Handbuch d. physiol. Optik," 1867, S. 364. 



4 Arch.f. d. ges. Physiol., Bonn, 1868, Bd. i. S. 375. 

 6 Arch.f. Physiol., Leipzig, 1882, Suppl., S. 113. 



6 Arch.f. Ophth., 1890, Bd. xxxvi. Abth. 1, S. 1. 



