CHAP. HI.] SIGHT. 915 



falling from what they do under bright daylight. In particular 

 we find that as the light diminishes red sensations and also yel- 

 low sensations disappear earlier than blue sensations. Hence in 

 dim lights, as those of evening and moonlight, blues preponder- 

 ate, reds and yellows being less obvious, whereas in bright lights 

 yellows and reds become prominent. 



On the other hand, if we test our sensitiveness to different 

 colours in a different way we get results which are opposed to 

 the above. If for instance we determine the distance at which 

 we cease to recognize the colour of a piece of coloured paper, say 

 1 cm. square, we find that the blue goes first, then green and 

 next yellow, red being recognizable at the longest distance, 

 though the difference between red and yellow is not very great. 

 It will be understood of course that in this experiment we are 

 dealing not only with diminished energy, with diminished ampli- 

 tude of the luminous waves, but also with a diminished area of 

 retinal stimulation. 



Or again, if we take the heating effects of rays of different 

 wave-lengths as a measure of their energy, we may determine 

 the amount of energy needed, in the case of the several colours, 

 to produce a given visual effect. When this is done it is found 

 that the rays in the green, about wave-length \ 530, are the most 

 effective ; from this part of the spectrum the efficiency declines 

 both towards the violet and the red. 



The three several methods lead to three different results, 

 the one teaches that blue, the other that red or yellow, and the 

 third that green is the colour to which the eye is most sensitive. 

 It would be hazardous to found important conclusions on any 

 of them. 



There are several other facts of considerable importance 

 bearing on the theory of colour vision, but it will be best to 

 consider these in connection with certain modifications of visual 

 sensations with which we shall have presently to deal. Mean- 

 while having acquired some general notions of visual sensa- 

 tions, we may turn from the study of the little we know 

 concerning the way in which these sensations originate through 

 retinal changes, to the study of the way in which light falling 

 on the retina gives rise to visual impulses. 



