Photometry of Lights of Different Colours. 369 



In the equality o£ brightness photometer (corresponding to 

 ordinary vision) the rods and cones act together, the former 

 prevailing at low illuminations. 



The new facts learned in the present investigation seem to 

 be in accord with the work of Kcenig and this suggested 

 mode of action of the flicker photometer. We must add to 

 the hypothesis, just reviewed, the assumption that the blue 

 sensation, although produced in the visual purple of the rods, 

 behaves in a similar manner to the red and green sensation 

 localized in the cones. It is not necessary to assume an 

 exaggerated sensitiveness in the peripheral rods because the 

 greater number of rods will alone account for that. We 

 have then at high illuminations colour vision, and response 

 to intermittent stimuli as represented by the logarithmic 

 relation discussed above. This state is due to cone action 

 and visual yellow decomposition. At a certain low illumi- 

 nation we cease abruptly to have blue vision. In its stead 

 we have colourless vision obeying an entirely different law 

 with respect to flicker and ascribable to the incomplete 

 breakdown of visual purple. In accordance with this 

 hypothesis is the fact that the violent change of direction in 

 the blue line of the log I-critical frequency diagram, is 

 coincident with the change in appearance from blue to gray. 

 Also the observation, in confirmation of the statement of 

 Koenig and others, that red light does not change to gray 

 but continues red until the lowest illumination possible 

 to make observations. The different behaviour of blue light 

 for a small or a large field at low illumination is also in accord 

 with the anatomical fact that the rods are missing in the 

 fovea, and with Koenig's observation of foveal blue blindness. 



Summary. 



1. Spectral luminosity curves obtained by the method of 

 critical frequency show a reversed Purkinje effect, but at 

 very low illuminations a true Purkinje effect, the latter 

 observed by Haycraft. 



2. A plot of critical frequencies against the logarithm of 

 the illumination for white light gives, as found by Porter, 

 two straight lines of different slope, which meet at about 

 2*5 I.U. The reversed Purkinje effect occurs above this 

 point, the true Purkinje effect below it. 



3. When separate colours are investigated and plotted in 

 the above manner, a set of straight lines of differing slope 

 results. 



4. At about 2*5| I.U. these lines in general change their 



