98 COLOUR VISION 



and the sensibility of the observer's eye to flicker for different colours 

 at a given speed. The Purkinje effect, and its reversal above 0'25 

 metre-candle, follow at once from these facts. 



In the flicker phenomenon with colours two causes of flicker are at 

 work, the colour element and the luminosity element 1 . Of these colour- 

 flicker ceases first. Hence the total flicker effect may be the resultant 

 of the two flicker sensations. Ives has shown that the flicker photometer 

 is largely influenced by the critical frequency phenomenon, but that 

 it does not obey the simple law which would follow were it a mere dove- 

 tailing of two pure flickers. 



Allen and others 2 found that the peripheral retina is more sensitive 

 than the fovea to flicker, as might be expected from its high sensitiveness 

 to movements. Ives, however, found that this result is only true for 

 momentary observation. Adaptation or fatigue sets in very rapidly 

 and then the periphery becomes less sensitive. The fovea is more 

 sensitive to red flicker, the periphery to blue, and this difference is more 

 striking at low intensities (Dow, Ives). 



If the comparison light is coloured or the stimulating area is sur- 

 rounded by white instead of black the equality of brightness method 

 produces irregular and unsystematic shifts and distortions of the spectral 

 luminosity curves, possibly owing to the increase in distracting psycho- 

 logical factors. Such changes produce no alteration in the luminosity 

 curves by the flicker method. A curve almost identical with the flicker 

 curve can be obtained by the equality of brightness method if it is 

 built up of small steps of slight hue-difference and with small areas. 

 By this so-called " cascade " method the differences in hue are made 

 so small that they do not disturb the judgment of brightness. 



When the areas of the luminosity curves by different methods are 

 compared it is found that the visual acuity method (v. p. 44) gives 

 a curve many (about five) times as great as the equality of brightness, 

 the flicker and the critical frequency curves, which agree much more 

 nearly. The enormous area of the visual acuity curve is due to the 

 chromatic aberration of the eye. It has been shown by Bell 3 and 

 Luckiesh 4 that the resolving power of the eye is much greater for 

 monochromatic than for complex light of the same hue. For a method 

 of measurement to be accurate it should conform to two axioms : things 

 equal to the same thing are equal to one another ; and the whole is 



1 Cf. Liebermann, Ztscli. f. Sinnesphysiol. XLV. 117, 1911. 



2 Cf. Lohmann, Arch. f. Ophth. LXVIII. 395, 1908. 



3 Elec. World, LVII. 1103, 1911. 4 Ibid. LVIII. 450, 1911. 



