AD APT A TION AND IND UCTION. 1059 



fatiguing light. If the eye is exposed to a coloured light, and then exposed 

 to its complementary light, the latter appears more saturated. 



Burch x has recently found that when light of great intensity is used, 

 characteristic fatigue effects are produced with four parts of the spectrum, 

 namely, red from A to B ; green in the neighbourhood of E; blue about half- 

 way between F and G ; and violet at and beyond H. In each case the eye 

 is found to be blind to the fatiguing colour ; thus after fatigue to green, the 

 red of the spectrum is seen to pass directly into the blue. Fatigue to yellow 

 caused disappearance of both red and green, so that only blue and violet were 

 left. After fatigue by three of the four parts of the spectrum only one colour 

 was seen. The red so obtained extended from A to a little beyond b ; the 

 green from C nearly as far as G. 



Persistent positive image. — Under certain conditions, a positive 

 after-image is seen, which persists for a considerable time. It differs 

 from the ordinary after-image in not undergoing the definite periodical 

 variations of the latter, although similar irregular variations may 

 be observed as it becomes faint. Such a positive image is seen with 

 the dark-adapted eye, and may be very well seen soon after waking. It 

 occurs with very short stimulation. 



The most marked positive image is that which occurs after very 

 intense stimulation. It is bright and well-defined when the eyes are 

 closed, but appears as a dark cloud when seen against a bright surface. 

 Its duration depends on the intensity of the stimulus, and it may persist 

 for minutes or hours. In its later stages it may become coloured, and 

 may also undergo irregular variations. In cases of very intense stimula- 

 tion, a similar phenomenon has been observed to persist for very long 

 periods, and even for life ; and there does not appear to be any clear 

 distinction between this definitely pathological phenomenon and the 

 ordinary persistent image. The latter possibly only occurs when the 

 stimulation has been sufficiently intense to disturb the normal functions 

 of the retina. 



Another phenomenon which has been observed with strong stimu- 

 lation of the retina, is coloured waning of the after-image (farbiges 

 Abklingen). The image of white light may be seen to appear in a 

 succession of colours, beginning with bright greenish blue, passing on to 

 blue, violet, pink, and then changing to a dirty orange and yellow-green. 

 The exact nature of the coloration varies with the intensity and dura- 

 tion of the stimulus. The colours have been referred to differences of 

 persistence (p. 1068), and may also in some cases be connected with 

 the colour adaptation of the retina. 



Corona of after-image. — The after-image change in a retinal area 

 influences and is influenced by the changes going on in surrounding 

 parts of the retina. The most marked instance of this is the 

 corona of an after-image. If a white patch on a black ground is 

 fixed, the black after-image will be seen to be surrounded by a light 

 halo. If watched, this halo may be seen to vary ; at one moment it is a 

 narrow border, at the next it broadens out, and undergoes periodical 

 changes analogous to those of the after-image itself. If one fixes two 

 white patches separated from one another by a narrow black strip, the 

 part of the after-image corresponding to the strip appears very bright, 

 and may be regarded as due to the superposition of two halos. In he 



1 Joum. Physiol., Cambridge and London, 1898, vol. xxii. p. xii. and Phil. Trans., 

 London, 1899, vol. cxci. p. 1. 



