PSYCHOLOGICAL SCIENCE 73 



While intense radiant energy, such as powerful 

 direct sunlight, affects the whole surface of our body, 

 and gives to our minds the sensation of heat, the 

 sensation of light is excited only when radiation of 

 a certain type falls on our eyes. Their sensitiveness 

 to radiation within a limited range is extraordinarily 

 great, an infinitesimal amount of energy being enough 

 to produce the characteristic sensation ; but the kind 

 of radiation which is effective is strictly limited, 

 lying between definite limits of frequency of vibra- 

 tion of light waves from about 400 to 800 millions 

 of millions per second, the greatest sensitiveness 

 being about the middle of this range, in the yellow. 



The sensations of light may be divided into colour- 

 less sensations a white to grey and black linear 

 series and colour sensations which are more com- 

 plex, and must be represented by some closed figure 

 such as a circle or triangle. Beginning with the 

 sensation of red, we may pass by insensible grada- 

 tions either towards orange or purple. If the former, 

 we reach successively, orange, yellow, green, blue, 

 violet, purple, and so complete our circuit and return 

 to red. If these colours be mixed with white or 

 grey, we get less pure shades. If red be mixed with 

 white or light grey, we get rose or pink ; if with dark 

 grey or black, the result is brown. 



The sensation of white may be excited either by a 

 mixture in the eye of all coloured lights, as in sun- 

 light, which contains all the colours of the spectrum, 



