360 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



We must next consider certain facts which seem to bear in- 

 directly upon the problem under examination ; of these the 

 most important are connected with the condition of total colour- 

 blindness. 



Total colour-blindness is almost always a congenital defect, 

 and is characterised, apparently, by a complete absence of colour 

 perception in the ordinary sense. A person in this state may 

 see a spectrum merely as a grey strip unequally bright in the 

 different regions which seem to us of various colours. To describe 

 the sensations of a second human being is, of course, an impossi- 

 bility ; but perhaps we may say, for the sake of comparison and 

 without pretending to any real precision, that a totally colour- 

 blind man receives from a coloured print impressions similar to 

 those excited in ourselves by an uncoloured one. The following 

 summary of observations by Hering ( 12 ) will give the reader a 

 tolerably clear impression of the facts. 



The subject was a man of twenty years, whose colour vision 

 had always been abnormal. He said that he could read without 

 difficulty, provided the light were not too intense, but that his 

 eyes were readily fatigued by bright illumination. In twilight his 

 vision was especially good, particularly if the light were very 

 feeble. On examination, the following results were obtained. No 

 objective changes were detected with the ophthalmoscope, nor was 

 any part of the retina insensitive ; there was no scotoma. His 

 power of distinguishing two spots unequally bright physically 

 was much below that of a normal person, in bright light ; in a dark 

 room, it was much superior. Working with a spectrum, it was 

 found that the area of red which produced any sensation was 

 much diminished ; there was shortening of the red end, and those 

 parts which were effective seemed less bright than to a normal 

 eye. The violet end, on the other hand, was not shortened, and it 

 seemed relatively brighter than to the normal " light " eye, while 

 the region of maximum brightness was in the neighbourhood 

 of the Fraunhofer E and C lines. Brightness matches between 

 coloured sectors and mixtures of black and white gave the results 

 indicated in the table, which contains comparative values for 

 the normal " light " eye. It is to be noted that the colour- 

 blind's matches were valid, i.e. good matches, for a normal 

 "dark" eye. 



