THE BRIGHTNESS OF COLOUR SENSATIONS. 1079 



blue becoming the brighter. This is a special case of a more general 

 phenomenon, which has since been named after Purkinje, namely, that 

 with decrease of the intensity of the spectrum, the short wave end 

 becomes relatively brighter. The question how far this change is a 

 function of the altered intensity as such, and how far of the altered con- 

 dition of adaptation of the eye, will be considered later. 



It has already been stated that, with the exception of red, all colours 

 appear colourless at a low intensity, and Purkinje's phenomenon attains 

 its maximum effect when the intensity has been so far reduced as to 

 produce a colourless spectrum. The brightest part of the spectrum is 

 then found to be in the green at E, or slightly on the violet side of E y 

 instead of about D, as at ordinary luminosities. The fact that the 

 interval between the absolute and chromatic thresholds is greatest for 

 green, stands in relation to this phenomenon. Further, the red end of 

 the spectrum is shortened, and is relatively much darker than in the 



700 



76? 



FIG. 390. In this figure the red end of the spectrum is on the right- 

 hand side, the blue on the left. The ordinates show the number of 

 revolutions of a semi-disc per second which were found necessary to 

 extinguish nicker. The upper curve represents a bright spectrum, 

 the lower a dark spectrum, and the middle curve one of intermediate 

 intensity. Haycraf t. 



ordinary spectrum, while the violet end is relatively brighter. The 

 curve of luminosity, as determined by flicker, is given in Pig. 390 

 together with the curves of luminosity at ordinary intensities, and in 

 an intermediate condition. 



The order in which the colours disappear on lowering the intensity 

 of the spectrum is said to be that yellow and blue go first, violet then 

 becoming invisible, while green persists longest. If the intensity of a 

 spectrum is increased above the ordinary limits, it also becomes colour- 

 less, and under these circumstances yellow and blue persist, after red, 

 green, and violet have disappeared. 



The loss of colour at high or low intensities does not take place 

 suddenly, but is preceded by a gradual decrease of saturation. Hering 

 has pointed out that if a red and blue be compared while the intensity 



