186 COLOUR VISION 



to simultaneous contrast effects, but these are spread over the whole 

 spectrum with the exception of yellow (compare the partial deuter- 

 anopes). The contrast-responding field is equal in luminosity to that 

 of the contrast-exciting field. Contrast extends over a larger area and 

 commences more rapidly than in the normal. It is strongest in the 

 green, and is greatest for small fields except towards the violet end. 

 Colour-fatigue sets in more quickly with them, as with the partial 

 deuteranopes. The two groups are thus easily confused. The normal 

 variations in the Rayleigh equation — matching D light with mixed 

 thallium- and lithium-line lights — are more restricted in one group 

 of partial deuteranopes than in the partial protanopes. 



It has been found that the partial dichromats vary relatively 

 little in their individual matches for the sodium line. If, however, 

 they match a wave-length materially shorter than 589 /z/x their matches 

 become very indefinite and variable. Considerable quantities of red 

 can be added without upsetting the match. On the other hand, for 

 536 /x/x for example, the red can be reduced almost to zero without 

 the fields ceasing to be at any rate nearly equal. These are extreme 

 cases of partial deuteranopia, and they therefore show very defective 

 sensibility for colour difl'erences in the neighbourhood of 570 — 535 /x/x. 

 Similarly there are extreme cases of partial protanopia in which pure 

 yellow and especially lights of somewhat greater wave-length appear 

 almost the same as the spectral red. 



CHAPTER IV 



MONOCHROMATIC VISION 



There are certain persons who appear to have no perception of 

 colour ; their vision is monochromatic. The condition is congenital, 

 and is known as total colour blindness or achromatopia. The subjects, 

 apart from the colour defect, usually have very bad central vision 

 (less than -f^), marked photophobia, and nystagmus. Several members 

 of a family (11 groups) may be affected either with the same condition 

 or less commonly with other forms of dyschromatopia. Consan- 

 guinity (five cases) and heredity appear to be factors in the incidence 

 of the disease. Males are afi^ected about twice as often as females. 

 Nettleshipi has published 10 pedigrees containing 34 cases affected : 

 18 males, 15 females, and 1 of unrecorded sex. 



1 Trans. Ophlh. Soc. xxix. cxci, 1900. 



