86 COLOUR-SIGHT AND COLOUR-BLINDNESS 



tetrachromic ; three, trichromic ; two, dichromic, and the totally 



colour-blind monochromic. The order on which, according to 



him the colours are evolved, may be seen in this table : 



Position of colour in spectrum 12 3 4 5 6 



Name of colour Red : Orange : Yellow : Green : Blue : Violet : 



Order of evolution 16 4 3 5 2 



And when there is a paucity of colours recognised, the missing 

 one is the latest in development, thus — a pentachromic would 

 not recognise orange ; a tetrachromic, blue ; a trichromic would 

 mix up the yellows and greens ; and " a dichromic confuse 

 red, orange, yellow and yellow-green on the one hand and 

 blue-green, blue and violet on the other." 



In illustration of this. Green has published some ten (10) 

 coloured spectra, which he supposes represent seven (7) different 

 types of colour perception — from the achromic or monochromic 

 to those who can see seven or six colours in the solar spectrum, 

 the hexachromic or perfect colour-sighted. 



Abney also issued coloured charts to represent a normal 

 spectrum, and one each as seen by the green-, the red-, and the 

 violet-blind. * 



But it does not strike me that the authors agree or that 

 their charts are a true solution of the colour difficulty. 



Colour Blindness, 



Or Daltonism, as it was for a long time called, because John 

 Dalton (1766-1844), an English chemist and physicist, himself 

 colour blind, described his defects of vision (1794). 



A capital article on the Life and Work of John Dalton 

 appears in the " British Medical Journal " of the 16th of May, 

 this year, and an extensive excerpt from his papers on Colour 

 Blindness is given in Dalton's own words. 



When a person can see seven or six colours in the spectrum 

 he is admittedly colour perfect ; when he can see five he might 

 be classed as fairly normal sighted ; but when he perceives only 

 four or fewer or none there is a marked defect in his colour 

 perception and he may be partially or completely colour-blind. 



The usual classification is — 



I. — Total Colour-Blindness. Achromatopsia. 



When all intensities of lightness and darkness may 

 be thoroughly recognised, but no colour per- 

 ceived. 



