778 COLOUB-BLIXPXESS. 



sensibility to light and the length of the spectrum are retained ; the brightest part 

 in this case, as in the normal eye, is in the yellow (Hering). 



397 COLOUR-BLINDNESS AND ITS PRACTICAL IMPORTANCE. 



Causes. By the term colour-blindness (dyschromatopsy) is meant a pathological 

 condition in which some individuals are unable to distinguish certain colours. 

 Huddart (1777) was acquainted with the condition, but it was first accurately 

 described by Dalton (1794), who himself was red-blind. The term colour-blindness 

 given to it by Brewster. 

 The supporters of the Young-Helmholtz theory assume that, corresponding to the paralysis 

 of the three colour-perceiving elements of the retina, there are the following kinds of colour- 

 blindness : _. , ... . 



1. Bed-blindness. 2. Green-blindness. 3. Violet-blindness. 



The highest degree being termed complete colour-blindness. 

 The supporters of E. Hering' 8 theory of colour sensation distinguish the following kinds : 



1. Complete Colour-blindness (Achromatopsy ). The spectrum appears achromatic; the 

 position of the greenish-yellow is the brightest, while it is darker on both sides of it. A 

 coloured picture appears like a photograph or an engraving. Occasionally the different degrees 

 <f light intensity are perceived in one shade of colour, e.g., yellow, which cannot be compared 

 with any other colour. O. Becker and v. Hippel observed cases of unilateral congenital com- 

 plete colour-blindness, whilst the other eye was normal for colour-perception. 



2. Blue-yellow Blindness. The spectrum is dichromatic, and consists only of red and green. 

 The blue-violet end of the spectrum is usually greatly shortened. In pure cases only the red 

 and green are correctly distinguished (Mauthner's erythrochloropy), but not the other colours. 

 Unilateral cases have been observed. 



3. Bed-green Blindness. The spectrum is also dichromatic. Yellow and blue are correctly 

 distinguished ; violet and blue are both taken for blue. The sensations for red and green are 

 absent altogether. There are several forms of this (a) Green-blindness, or the red-green 

 blindness, with undiminished spectrum (Mauthner's xanthokyanopy), in which bright-green and 

 dark -red arc confounded. In the spectrum yellow abuts directly on blue, or between the two, 

 at most, there is a strip of grey. The maximum of brightness is in the yellow. It is often 

 unilateral and often hereditary, (b) Bed-blindness (or the red-green blindness with undi- 

 minished spectrum, also called Daltonism), in which bright-red and dark-green are confounded. 

 The spectrum consists of yellow and blue, but the yellow lies in the orange. The red end of 

 the spectrum is uncoloured, or even dark. The greatest brightness, as well as the limit between 

 yellow and blue, lies more towards the right. 



4. Incomplete colour-blindness, or a diminished colour sense, indicates the condition in 

 which the acuteness of colour perception is diminished, so that the colours can be detected only 

 in large objects, or only when they are near, and when they are mixed with white they no 

 longer apj>ear as such. A certain degree of this form is frequent, in as far as many persons are 

 unalde to distinguish greenish-blue from bluish-green. 



Acquired colour-blindness occurs in diseases of the retina and atrophy of the optic nerve in 

 commencing tabes, in some forms of cerebral disease (p. 713), and intoxications. At first 

 green-blindness occurs, which is soon followed by red-blindness. The peripheral zone of the 

 retina suffers sooner than the central area. In hysterical persons there may be intermittent 

 attacks of colour-blindness (Charcot) ; and the same occurs in hypnotised persons (p. 686). 



H. Cohn found that, on heating the eyeball of some colour-blind persons, the colour-blind- 

 ness disappeared temitorarily. Occasionally in persons without a lens red vision is present, and is 

 due to unknown causes. Percentage. Holmgren found that 27 per cent, of persons were 

 colour-blind, most being red and green blind, and very few violet blind. 



Limits of Normal Colour-blindness. The investigations on the power of colour perception 

 in the normal retina are best carried out by means of Aubert-Forster's perimeter, or that of 

 M'Hardy ( 395). It is found that our colour perception is complete only in the middle of the 

 ji> hi <>f vision. Around this is a middle zone, in which only blue and yellow are perceived, in 

 which, therefore, there is red blindness. Outside this zone, there is a peripheral girdle, where 

 then- is complete colour-blindness ( 395). Hence a red-blind person is distinguished from a 

 person with normal vision, in that the central area of the normal field of vision is absent in the 

 former, this being rather included in the middle zone. The field of vision of a green-blind 

 person differs from that of a person with normal vision, in that his peripheral zone corresponds 

 to the intermediate and peripheral zones of the normal eye. The violet-blind person is dis- 

 tinguished by the complete absence of the normal peripheral zone. The incomplete colour- 

 blindness of these two kinds is characterised by a uniformly diminished central field. [When 

 very intense colours are used, such as those of the solar spectrum, the retina can distinguish 

 them quite up to its margin (Landolt).] 





