RELATIONSHIP BETWEEN STIMULUS AND SENSATION 565 



mality in the response of the retina to colours. Because of this the method 

 of colour mixtures forms a very valuable technique for the investigation of 

 colour-blindness. 



THE FLICKER METHOD. The majority of observers can only 

 obtain consistent measurements of intensity with the above method 

 when the colours of the two patches are exactly alike. Thus it 

 is difficult to adjust a green light to be of equal apparent brightness 

 (luminosity) with a red because the difference in colour makes the judgment 

 of brightness inaccurate. Abney found that in his own case practice 

 greatly increased the certainty with which the measurement could be 

 made. A more reliable technique is given by the flicker method 

 (see page 559). The two patches are viewed through a rotating sector, 

 the speed of rotation of which can be controlled. TheHnfensity of one of them 

 is now adjusted so that, when the speed of the sector is altered, both commence 

 to show and to cease showing flicker at the same time. By applying this 

 method to the colour- mixing apparatus the luminosity of the different parts 

 of the spectrum can be determined. The curves obtained at different lumino- 

 sities have already been given in Fig. 286. Tested by this method different 

 observers show individual peculiarities, which amount in some cases to a 

 greatly diminished perception of a certain part of the spectrum. Some of 

 the types met with will be described later. 



