A DAPTA TION AND IND UCTION. 1057 



twenty minutes, but continued to increase slightly for two hours, when 

 it was thirty-five times greater than at first. Charpentier x found the 

 rapidity of the increase corresponded to that described by Aubert, but 

 described the amount of increase as much greater, the eye completely 

 adapted to darkness being able to see light one thousand (in one case 

 two thousand five hundred) times less intense than after exposure to 

 bright light. In adapting the eye to various illuminations, he found 

 that the sensitiveness increased much more rapidly as complete darkness 

 was approached. According to Charpentier, the increase of sensitiveness 

 is wholly due to the retinal condition, and is unaffected by differences 

 in the size of the pupil. Adaptation to bright illumination after being 

 in the dark, takes place much more rapidly than adaptation to darkness. 



One effect of dark adaptation is to increase the vividness of the 

 intrinsic light of the retina, and this increase may introduce difficulties 

 in ascertaining the threshold of sensibility for light. The two effects do 

 not necessarily increase 'pari passu, and Aubert thought it probable that 

 they were independent of one another, the intrinsic light being of central 

 origin, while the increase of sensitiveness is due to the condition of the 

 retina (p. 1096). 



The retina is not only capable of adaptation to difference of lumin- 

 osity, but also to differences of colour. The light which enters the eye 

 is always more or less coloured. If coloured glasses are worn, the 

 retina becomes adapted to the colour, and objects after a time again 

 appear colourless. When the glasses are removed, objects appear 

 coloured in the colour complementary to that of the glasses, and it is 

 not till the eye has become again adapted to white light that objects 

 appear colourless. Brlicke 2 has shown that in the normal condition our 

 eyes are always more or less adapted to red, owing to the light which 

 passes through the iris and sclerotic, the effect being especially great 

 with lateral illumination. It is then much more noticeable, because 

 different in the two eyes. A mixture of red and green which is 

 yellow in daylight, will appear reddish to the eye adapted to the dark. 

 This source of error may be minimised by wearing a black, funnel- 

 shaped shade to keep off lateral light when experiments in colour vision 

 are carried out in a lighted room. 



After-images. — When the adaptation is local, the appearances 

 which occur are known as after-images, and may be seen in the 

 absence of any objective light stimulus. When a limited surface is 

 accurately fixed, it may appear to become brighter or darker according 

 to circumstances ; if coloured, it may appear to change in colour-tone, 

 brightness, or saturation. If after a certain period of fixation the eyes 

 are closed or directed to a grey surface, larger than the fixed surface, a 

 part of the visual field will be seen, corresponding in form to the latter. 

 This after-image may be of the same brightness as the original, or the 

 reverse, and is known as positive or negative, in the same sense as in 

 photography. If coloured, it may be of the same colour as the original, 

 or it may be complementary. The term negative is often applied to the 

 latter, but it is better to limit that term to denote brightness, and to 

 speak of coloured images as homochromatic or complementary. The 

 after-image of a colourless surface may be coloured, usually in some 

 variety of green, and that of a coloured surface may not be exactly comple- 



1 ' : La lumiere et les couleurs," Paris, 1888, p. 150. 



2 Ann. d. Phys. u. Chan., Leipzig, 1851, Bd. lxxxiv. S. 418. 



VOL. II. — 67 



