366 Mr. H. E. Ives on the 



The slopes of the straight lines, obtained by plotting, as in 

 fig. 3, are subject to variations, in general o£ such a nature 

 as would be caused by variations in the size of the field, 

 indicating a changing balance between rods and cones. A 

 rather curious difference in the setting depends on whether 

 the variable be speed or brightness. If the slit- width for 

 any colour is held constant the critical speed may be found by 

 several settings. If this speed is held and the brightness 

 varied by changing the slit till flicker vanishes, the value 

 obtained from settings for slit-width may differ as much as 

 15 or 20 per cent, from the previously fixed value, an effect 

 apparently acting one way at one period and another way at 

 another. This disturbance affects all the spectral colours to 

 the same extent (as nearly as can be judged) so that a curve 

 of larger or smaller area results depending on the method 

 of setting. 



In view of this fact the critical frequency curves of fig. 1, 

 curves A to D were made by speed settings alone (except D). 

 First the critical speed for the standard lamp illumination 

 was determined ; then, with the slit set at the value given by 

 the previous flicker photometer setting, the critical speed 

 for the colour was determined. From the log I-critical 

 speed relations previously determined the equivalent change 

 in slit-width was read off. The relative values for the 

 different colours, that is the shape of the curves and the 

 Purkinje shift, were as previously obtained by the procedure 

 described above, but these curves are given as being freer 

 from objection. 



Great care is taken in the present paper to draw only 

 such conclusions as are warranted by the accuracy of the 

 method. 



The Areas of the Curves. 



Because of the small sensibility of the critical frequency 

 method and its susceptibility to obscure disturbing factors, 

 it is hard to draw very definite conclusions from the difference 

 of area shown between certain of the critical frequency 

 curves and the flicker and equality of brightness curves. 

 Suffice to say that such differences are apt to occur and that 

 the previously found differences of area between the flicker 

 and equality of brightness curves again appear *. These 

 are sufficient to show the futility of concluding that different 

 photometric methods give identical results simply because 



* In fig. 1, curves A, the equality of brightness area is smaller than 

 the flicker, a condition not previously found at this illumination 



