580 PHYSIOLOGY 



Cases of colour blindness, when tested by the above methods, are 

 found to show every possible variation between complete blindness 

 and slight impairment of the colour sense. Their classification is as a rule 

 complicated by the fact that cases are usually described in te,rms of one of 

 the theories of colour vision. Most varieties of colour blindness are inherited, 

 and are commoner in men than in women. But it may also be acquired, 

 as explained above, in poisoning by alcohol and tobacco. Cases of colour 

 blindness may be grouped as follows : 



1. Cases in which the cone mechanism of the retina is not functioning. 

 The patient is found to be colour and day blind, red is not seen at all, while 

 the other colours are seen as different shades of grey. Vision at night is 

 good, vision by day is complicated by the fact that the patient must not 

 expose his eyes to a bright light, for otherwise his visual purple will 

 become bleached and his rod apparatus therefore cease to function. Owing 

 to the absence of rods from the fovea, this part of the retina is blind. 

 Visual acuity is low therefore. When tested by the flicker method his 

 luminosity curve is found to correspond to that of twilight vision. His 

 condition may be improved by using neutral tinted glasses fitted with a sky 

 shade. 



2_ Cases in which the cone apparatus is apparently normal, that is to 

 say, there is no avoidance on the part of the patient of strong light, no 

 diminished visual acuity, no foveal blindness and no inability to see red 

 light. Yet there is absolute inability to recognize all colours, any one part 

 of the spectrum being able to *be matched by any other. Tests by means 

 of the flicker method show a luminosity curve which corresponds to that 

 of day vision. It would seem clear that the retinal apparatus is in every 

 way normal ; one is therefore forced to the conclusion that the defect concerns 

 the brain centre which subserves the appreciation of colour. This view is 

 supported by the fact that, between this extreme type and normal colour 

 perception, there are a large number of cases which show various grades 

 of defect. Some, for example, see two colours at the ends of the spectrum 

 only, the intermediate portion being a neutral colour ; others see three only, 

 at red, green and blue, and so on. Since in all these cases the cones are appar- 

 ently normal, there would appear to be a parallel with cases in which there 

 is no trace of deafness, and yet there is an inability to appreciate harmony 

 or to tell when two notes are in tune. In both types of cases it would seem 

 that the higher centres of perception and appreciation are either absent or 

 are undeveloped. As might be anticipated therefore, instruction and practice 

 at colour naming and colour matching benefit a certain number of these 

 cases, so that it is sometimes found that after such instruction the less abnormal 

 cases are not readily detected. If however they are tested in a poor light, 

 they are found to make mistakes which a person with normal vision would 

 not commit. But these are the circumstances under which signals have 

 frequently to be recognised, and it is for this reason that the lantern test 

 with its modifying 'glasses is so valuable. 



:*. Cases in which certain parts of the spectrum are not seen at all. This 



