52 COLOUR VISION 



stimulated, and the relations between sensibility and the area stimulated 

 are different in the light- and dark-adapted eye, and also in the foveal 

 region and the periphery. For foveal vision the sensibility is pro- 

 portional to the area stimulated (Ricco, Loeser 1 ). In the dark-adapted 

 periphery the sensibility is proportional to the square root of the area 

 stimulated 2 , but only for composite -white light and objects subtending 

 a visual angle of 1 to 10. Above 10 the sensibility rises more slowly. 

 The rise is still less even for smaller angles with red light. The rise of 

 the curve of sensibility in dark adaptation therefore varies with the 

 size of the area of retina stimulated and with the nature of the light. 

 In the light-adapted eye there is no definite relationship between the 

 rise of sensibility and the size of the visual angle. 



In dark-adapted eyes binocular summation of stimuli occurs, so 

 that the sensibility is about twice as great with both eyes as with 

 only one (Piper), though individual variations occur. In light-adapted 

 eyes no such binocular summation occurs (Fechner 3 ), but care must be 

 taken that there is good light adaptation, and one eye must be covered 

 for only a very short period, otherwise partial dark adaptation occurs. 

 In this respect there is a noteworthy analogy to the effects of the size 

 of the area of the retina stimulated : with complete light adaptation the 

 stimuli to different parts of the retina are not summated, given that 

 the visual angle exceeds a certain (small) size, nor are the stimuli to the 

 two eyes summated, whereas in the condition of dark adaptation both 

 summations occur. 



CHAPTER II 



SCOTOPIA OR TWILIGHT VISION 



It will be readily appreciated that complete dark adaptation rarely 

 occurs under normal conditions of life. Scotopia is the condition of 

 vision in which there is a relatively high degree of dark adaptation. 

 It will be best to consider the conditions of vision after prolonged stay 

 in a feebly lighted room. If now coloured objects are viewed under 

 feeble illumination the colours cannot be distinguished, but all appear 

 to be of various shades of grey. The eye is totally colour-blind. 



1 Parsons, Roy. Lond. Ophth. Hosp. Rep. xix. 114, 1913. 



2 Piper, Ztsch. /. Psychol. u. Physiol. d. Sinnesorg. xxxn. 98, 1904. 



3 v. Helmholtz, 3rd ed. n. 287. 



