372 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



basal mechanism is similar to that associated by the theory with 

 twilight vision and uncomplicated by the existence of a second 

 type of reaction ? The subjects of total-colour-blindness appear 

 to enable us to answer the question affirmatively. We found that 

 the brightness judgments of these people agree well with those of 

 normal men in a state of dark adaptation ; that there is evidence 

 in such cases of diminished or absent foveal sensitivity, bad fixation, 

 inferior acuteness of vision, nystagmus, central scotoma (some- 

 times) and abnormally good vision in twilight. 



Conversely, are there any cases in which the hypothetical day- 

 light mechanism alone reacts ? 



Parinaud ( 10 ) has investigated several cases of "night-blindness " 

 or hemeralopia. He found that in such, vision was of the foveal 

 type ; the colour sense was normal, but the spectrum shortened at 

 the violet end ; responsiveness to short waved light was abnormally 

 low. The investigations of Messmer ( 2e ) and others make it pro- 

 bable, however, that the condition of night-blindness is not simple, 

 different forms and degrees being classified under the same heading. 

 In some cases, dark adaptation is very slowly induced, but after 

 a sufficiently long interval is normal in degree. In others some 

 adaptation comes on in a normal time but is of inferior extent. 

 It is important to bear these cautions in mind because night-blind- 

 ness is markedly heritable and has been used, in the writer's opinion 

 illogically, as an argument in favour of the validity of the Mendelian 

 theory of inheritance. For our present purpose, we must admit 

 that night-blindness does not afford us so much information re- 

 specting visual processes as the apparently opposite condition of 

 total-colour-blindness. Provisionally, we may, perhaps, say that 

 the latter condition is consistent with the activity of the hypothe- 

 tical twilight mechanism functioning by itself, while some examples 

 of the former peculiarity suggest that the daylight mechanism alone 

 exists. But it is to be remembered that scarcely any theory, how- 

 ever absurd, which has been advanced in explanation of a difficulty 

 in sense physiology has failed to obtain the support of some patho- 

 logical phenomena which have been tortured into a semblance of 

 agreement with its postulates. 



The complex results in sensation which follow the application 

 of short or moving stimuli to the retina have perhaps confused 

 some readers ; let us see whether our hypothesis is capable of 

 arranging them in an orderly manner. 



