COLOUR-BLINDNESS. 1095 



occurs in two chief forms ; and it may begin by peripheral constriction 

 of the colour fields, the defect spreading inwards, or as a central scotoma 

 spreading outwards. In both cases red and green seem to be almost 

 always the first colours to go, blue and yellow remaining. In acquired 

 total colour-blindness the evidence seems to be in favour of the maxi- 

 mum brightness being at D, and not at E> as in the congenital form. 



One distinction of importance in localised defects of retinal sensibility 

 may be mentioned here. These defects usually occur either as cases of 

 hemianopia or as scotomata, usually central. In some cases there is 

 complete absence of vision in the defective part of the retina ; the 

 condition resembles that of the blind spot ; and often the patient may 

 only be made aware of his defect by special means, although in the case 

 of hemianopia the defect is usually marked enough to attract attention 

 spontaneously. In other cases the defective part of the field may be 

 occupied by a cloud (Schwarzsehen). This appears to be common in 

 central scotoma, but does not seem to have been recorded in hemianopia. 



Santonin vision. Abnormalities of colour-vision are found under 

 certain other conditions. A normal individual, who has worn coloured 

 spectacles, or been in coloured illumination for a long time, will 

 confuse colours in a way that may resemble colour-blindness. More 

 important is the condition produced by certain drugs, and especially by 

 santonin. This produces yellow vision in bright light, and violet vision 

 in the dark, while in bright light, black objects often appear to have 

 violet borders. The violet has been regarded as an after-image effect, 

 but it seems to occur when the eyes have been altogether kept in the 

 dark after taking the drug. The violet end of the spectrum is shortened. 

 There does not appear to be any marked change in other parts of the 

 spectrum, nor anything corresponding to the neutral band of yellow- 

 blue blindness ; in fact, some observers have described the yellow of the 

 spectrum as more intense than normal. Haidinger's brushes, and 

 especially the yellow part, become more distinct than normally ; and a 

 definite yellow spot may be seen at the fixation point, apparently corre- 

 sponding to the macula (Konig). 1 Schultze 2 supposed that the santonin 

 effect depends upon coloration of the macula, and in favour :-of this is 

 the increase in the distinctness of Haidinger's brushes, which probably 

 depend upon the yellow spot ; while, according to Preyer, 3 the shortening 

 of the violet end of the spectrum only occurs with direct vision, the 

 violet of the spectrum again becoming visible when observed indirectly. 

 Eose, 4 on the other hand, advocated the view that it depends on an affec- 

 tion of the nervous system, and failed to find coloration of any part of the 

 eye sufficient to account for the phenomena. It seems probable that 

 the condition is of a different nature to yellow-blue blindness. 



Picric acid is said also to produce yellow vision, and yellow vision 

 has been described in jaundice. 



Night-blindness and the vision of albinos. Night-blindness is a 

 condition of theoretical interest. It occurs in several abnormal conditions 

 of the retina and choroid, but is especially well marked in retinitis 

 pigmentosa, in which the retinal pigment layer appears to be chiefly and 

 primarily affected. It also occurs after exposure to too strong light, 



1 Centralbl. f. prakt. Augenh., Leipzig, 1888, S. 353. 



2 "Ueber den gelben Fleck der Retina," Bonn, 1866. 



3 Arch.f. d. ges. PhysioL, Bonn, 1868, Bd. i. S. 299. 



4 Virchows Archiv., 1860, Bd. xviii. S. 29. 



