RECURRENT VISION 97 



phenomenon) by the equality of brightness method ; by the flicker 

 method it shifts towards the red. On the other hand, decrease of the 

 area stimulated at low intensities shifts the maximum of luminosity 

 towards the red by the equality of brightness method, towards the blue 

 by the flicker method. Ives found the relative positions of the two kinds 

 of spectral luminosity curves generally different. They differ most in 

 position at low illuminations with large areas ; least at high illumina- 

 tions with small areas. The mean curves of several observers show 

 close agreement in the position of the maxima and the shape of the two 

 curves at high intensities, but the areas of the curves are not equal. 

 At low illuminations all observers agree in showing the Purkinje and the 

 reversed Purkinje effects. 



Haycraft, by the critical frequency method, obtained a pronounced 

 Purkinje shift at low intensities. Ives obtained the reverse effect, 

 except at very low intensities (0"5 metre-candle), when he confirmed 

 Haycraft's result. Ives sought an explanation in Porter's change in the 

 logarithmic rates at which critical frequency varies with the illumination. 

 In Porter's equation 



n = k . log I + k' 



k has a different value above and below 0*25 metre-candle. If the 

 critical frequencies are plotted against the logarithms of the illumination 

 for white light two straight lines of different slope, which meet at about 

 0"25 metre-candle, are obtained. The reversed Purkinje effect occurs 

 above, the true Purkinje effect below this point. 



When separate colours are investigated and plotted in the same 

 manner, a set of straight lines of differing slope results. The most 

 remarkable curves are those for red (650 ^/x) and blue (480 /xju). The 

 former maintains its direction unchanged ; the latter suddenly changes 

 from a diagonal to a horizontal, i.e., the critical frequency becomes a 

 constant, independent of the (low) illumination. The curves for other 

 colours take an intermediate course. Hence Porter's law for white 

 light holds good for different colours if the values of the constants are 

 changed. 



i^==/iC,.log^, + /iC/, 



where F is the critical frequency, S^ is the slit-width, K^ is a constant 

 involving the relationship between critical frequency and intensity of 

 radiation for the individual eye for the colour in question and for the 

 size of the area stimulated, and K^' is a constant involving the quantity 

 of energy emitted by the source, the dispersion, etc. of the instrument, 

 p. c. V. 7 



