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Colour Blindness
 

Most cases of congenital colour vision deficiency are characterized by  a red-green deficiency which . may be of two types:

 
First, a protan type which may be absoulte (protanopia) or partial (protanomalia), and Secondly, a deutan type which may be absolute (deuteranopia) or partial (deuteranomalia). In protanopia, the visible range of the spectrum is shorter at the red end compared with that of the normal, and that part of the spectrum which appears to the normal as blue-green, appears to those with protanopia as grey. The whole visible range of the spectrum in protanopia consists of two areas which are separated from each other by this grey part. Each area appears to those with protanopia as one system of colour with different brightness and saturation within each area, the colour in one area being different from that of the other. The red with a slight tinge of purple which is the complementary colour of blue-green appears also as grey.
 
 
In deuteranopia, that part of the spectrum which appears to the normal as green, appears as grey, and the visible range of the spectrum is divided by this zone into two areas, each of which appears to be of one system of colour. The visible range of the spectrum is not contracted, in contrast to protanopia.    Purple-red which is the comple- mentary colour of green appears also as grey. In protanomalia and deuteranomalia, there is no part of the spectrum which appears as grey, but the part of spectrum which appears to those with protanopia as grey, appears to those with protanomalia as a greyish indistinct colour, and likewise, the grey part of the spectrum seen by the person with deuter-anopia appears to those with deuter- anomalia as an indistinct colour close to grey.
Consequently, one of the peculiarities of red-green deficiencies is that blue and yellow colours appear to be remarkably clear compared with red and green colours. The application of this peculiarity to the test for colour vision deficiencies is the distinguishing feature of this series. In the congenital colour vision deficiencies, although very rare, there is total colour blindness which may be typical or atypical. The subject who suffers from typical total colour blindness shows a complete failure to discriminate any colour variations, usually with an associated impairment of central vision with photophobia and nystagmus.In the atypical total colour blindness, the colour sensitivity to red and green, as well as to yellow and blue is so low that only very clear colours can be perceived; but, except for the colour sensitivity, there is no abnormality in the visual functions.
 
Furthermore, a failure in the appreciation of blue and yellow may be termed tritanomalia if partial, and tritanopia if absolute, but, even if such cases do exist, they are extremely rare. The plates in this book are not designed for the diagnosis of such cases.