Photometry of Lights of Different Colours. 365 



frequency of about 12 cycles per second, while Haycraft 

 worked as low as 4 or 5 cycles per second. The explanation 

 may be sought in two facts : his work was done with the 

 eye in an extreme condition of dark adaptation brought on 

 by one hour's rest in the dark before observations, and a 

 very small photometric field was used. That the constant 

 frequency attained by blue light depends upon the condition 

 of the eye is shown by the different values in figs. 4 and 5, 

 while the work on small fields shows that the turning point 

 may be depressed by restricting the area of the retina. The 

 combination of these two factors is probably sufficient to 

 explain the different values of critical speeds at which the 

 same phenomena occur in the two investigations. Pecu- 

 liarities of the individual eyes, such as differences in the 

 relative proportions of rods and cones, might also be of 

 influence. 



Sensibility of the Critical Frequency Method. 



Photometry by the method or' critical frequency is con- 

 siderably less accurate than by either the equality of 

 brightness or the flicker method. From among the sets 

 of readings made near the close of the investigation one 

 day's observations on " white " light were selected at random 

 and studied with the following result : At a high illumination, 

 corresponding to 43 cycles per second, the average deviation 

 from the mean of twenty settings was 1*8 per cent, in speed ; 

 at 32 cycles per second, 1*9 per cent, in speed ; at 23 cycles, 

 1*9 per cent, in speed. These errors appear small, but when 

 expressed in terms of brightness instead of speed they are 

 found to be too large for accurate photometry. They 

 correspond respectively to 18 per cent., 14 per cent v and 

 10 per cent, in brightness, or five or ten times the mean 

 error by the flicker method. Settings made on different 

 days, or on the same day with the fatigued and unfatigued 

 eye, are apt to differ by two to three per cent, in speed. 



The critical frequency method is highly subject to dis- 

 turbances whose explanation is physiological or psychological. 

 Depending on whether the eye has been rested in diffused 

 light, in darkness, or by blindfolding, the absolute values of 

 the readings will be increased or decreased throughout the 

 spectrum *. Some experiments made during the period 

 covered by this investigation indicate that these changes 

 may affect an unsaturated colour more than a saturated one, 

 or less. This would cause the area of the luminosity curve 

 to be in error. 



* F. Allen, Phys. Eev. 1900 & 1909, loc. cit. 



