INTRODUCTION: COLOUR NAMES 159 



blue-blind, and this terminology has generally been adopted by phy- 

 sicists. Hering, on the other hand, regarded the first two groups as 

 variants of a common class, the red-green-blind, the third group being 

 due to abnormality of the mechanism subserving blue-yellow sensations. 

 v. Kries introduced terms for the three groups which were merely 

 descriptive and were prejudiced by no theory. I shall adopt his terms, 

 viz., protanopes, deuteranopes , and tritanopes, corresponding respectively 

 with v. Helmholtz' red-, green-, and blue-blind. 



In 1881 Lord Rayleigh 1 made an important discovery. He found 

 that many people with apparently normal colour vision require different 

 amounts of red or green in their colour mixtures from the majority. 

 His observations were confirmed by Bonders 2 and have since been 

 verified. As the colour system of these people is, like that of normal 

 people, a function of three variables, they may be conveniently termed 

 anomalous trichromats. 



Later researches tend to show that these do not constitute the only 

 type of abnormality of the trichromatic system. 



All the varieties hitherto mentioned are abnormal from birth. The 

 defect is congenital and incurable. The statistics are very unreliable. 

 In its grosser forms (dichromats) amongst civilised races it is said to 

 affect about 4 per cent, of the male population, and O4 per cent, of the 

 female ; but the tests from which these statistics are derived were often 

 crude. There is good evidence to show that colour blindness is hereditary 

 and that generally it is transmitted through the female, who is herself 

 not usually affected 3 . An unaffected male never carries colour blindness, 

 but an affected male sometimes transmits to his son ; consanguinity 

 of parents by intermarriage of cousins is rare 4 . Transmission 

 through several branches of a family is not uncommon, and several 

 siblings, usually of course male, of a childship are often affected. Nettle- 

 ship has published pedigrees of colour-blind families in which females 

 were affected 5 . Statistics are not available for any precise estimate of 

 the slighter defects of colour vision (anomalous trichromats), but they 

 are probably widespread and certainly occur in colour-blind childships. 



Differing from all these groups there are people who apparently see 

 all parts of the spectrum of one hue, the parts differing only in luminosity. 

 These are the total colour-blind or monochromats. 



1 Nature, xxv. 64, 1882. 2 Arch. f. Anat. 518, 1884. 



3 Cf. pedigree, in Nettleship, Trans. Ophth. Soc. xxvrn. 248, 1908 ; also bibliography 

 in Parsons, Pathology of the Eye, iv. 1413, 1908. 



4 Nettleship, Trans. Ophth. Soc. xxix. p. lx, 1909. 



5 Ibid. xxvi. 251, 1906. 



