96 COLOUR VISION 



that at the point of fusion the effective stimulus at any point of the 

 retina is to the maximum stimulus as the angle of the bright sector to 

 360°. The fact that the colour stimulus requires a finite time to produce 

 its maximum effect was confirmed by this method. The period during 

 which the sensation remains undiminished appears to decrease as the 

 time of stimulation increases, though within narrow limits of variation 

 one of these quantities is nearly inversely proportional to the other. 

 He found that the relation between the intensity of illumination (/) 

 and the number of revolutions {n) per minute at which a disc half white 

 and half black must be run in order that the flicker may just disappear, 

 the distance of the eye remaining constant, is 



n = k . log I + k' 



k and k' being constants. He therefore proved the geometrical- 

 arithmetical relationship between intensity and rapidity of stimulation. 

 The relation was found rigidly true for illuminations from 0"25 metre 

 candle to 12,800 times this value. For intensities below 0'25 metre candle 

 the constant k suddenly changed its value to practically half its former 

 value. We have here further evidence of the duplex mechanism 

 involved, the higher value of k applying to the photopic, the lower to 

 the scotopic mechanism. T. C. Porter also proved that the duration of 

 the undiminished sensation produced by different spectral hues depends 

 solely on the luminosity of the colours and not on their wave frequency. 



.Ives's researches have elicited several highly important results and 

 are specially valuable owing to the great care taken to secure accuracy 

 in his methods. The flicker method can be applied in two ways. In 

 one, that used by Haycraft, Ferry and Allen, the critical frequency 

 of alternation of the lights is measured, i.e., two lights are regarded as 

 being of the same luminosity when the flicker produced by rapid alterna- 

 tion of each with black disappears at the same speed of alternation. 

 The other method is that employed by T. C. Porter and generally used 

 in flicker photometry. By it two lights are regarded as being of the 

 same brightness when no flicker results on alternating one with the 

 other, the speed of alternation being such that the slightest change 

 of intensity of either light causes flicker. Ives found that the second 

 or ordinary method is more sensitive than the equality of brightness 

 method used by Abney and others {v. p. 44). The results are reproduced 

 with much greater constancy, since in the latter method psychological 

 factors influence the judgment in arriving at the results. On decreasing 

 the illumination the maximum shifts towards the blue (Purkinje's 



