242 



COLOUR VISION 



area. Fig. 7 1 shows his foveal and parafoveal curves. Unusual macular 

 pigmentation may be combined with approximate dichromatism. as in 

 Fig. 72, where partial deuteranopia coexists with excessive macular 

 pigmentation. 



Other methods can be employed for diagnosing approximate dichro- 

 matism. In addition to taking the ordinary equation, i.e., matching 

 the white of the source of light with a mixture of homogeneous red, 

 green and violet, a pure colour may be matched with a mixture of two 

 pure colours, as in the Rayleigh equation ; or a mixed colour, such as 

 that obtained by passing the white light of the re-combined spectrum 



120 



110 



100 



40 



30 



20 



10 



64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28 



oloo 



50IOOAU 



Pig. 71. Luminosity curves of a normal trichromat with average macular 

 pigmentation. + + + foveal ; O O O parafoveal. (Watson.) 



through a saturated solution of potassium chromate, may be matched 

 with a spectral colour. 



A saturated solution of this salt, f inch thick, will filter off nearly 

 all the blue in the white light, so that the light appears yellow. Now, 

 from about SSN 50 to the red end of the spectrum there is no measurable 

 amount of blue sensation present. If a slit is caused to traverse the 

 spectrum a position will be found where the light exactly matches 

 that transmitted through the chromate. For the normal eye it is at 

 SSN 49-6 (583/,1/z). If a partial deuteranope makes the match the 



