56 



COLOUR VISION 



As may be easily imagined there are many practical difficulties 

 about the observations. Before examining these and other such 

 experiments more in detail it will be well to consider briefly some of the 

 outstanding peculiarities of scotopia or twilight vision as compared with 

 photopia or daylight vision. 



400 460 620 680 640 700 760 



Fig. 17. Liiminosity curves determined by the flicker method (critical frequency readings). 

 The upper curve i-epreseuts a briglit spectrum, the lower a dark spectrum, and the 

 middle curve one of intermediate intensity. (Haycraft.) 



" At nightfall in the summer the order of disappearance of colour 

 may often be seen ; orange flowers may be plainly visible, yet a red 

 geranium may appear black as night ; the green grass will be grey when 

 the colour of the yellow flowers may yet be just visible. An early 

 morning start in the autumn before daybreak will give an ample oppor- 

 tunity of satisfying oneself as to the order in which colours gradually 

 reappear as daybreak approaches. Red flowers will be at the outset 

 black, whilst other colours will be visible as grey. As more light comes 

 from the sky the pale yellow and blue flowers will next be distinguished, 

 though the grass may still be a nondescript grey. Then, as the light 

 still increases, every colour will burst out, if not in their full brilliance, 

 yet into their own undoubted hue^." It is instructive with good dark 

 adaptation in a dull light, to arrange different coloured cards, about 



^ Abney.. Colour Vision (Tyndall Lectures), p. 107, 1895. 



