Photometry of Lights of Different Colours. 157 



the flicker method because of its greater definiteness. It 

 might be expected too that the methods would agree. Yet 

 apparently this last is not the case. The flicker method 

 has not been so generally used as its convenience would 

 recommend, because numerous disagreements have been 

 recorded between its renderings and those of the longer 

 established method of direct comparison or equality of 

 brightness. The explanation of these differences is most 

 likely found in the different conditions which have obtained 

 when the characteristics of the two methods have been 

 studied, and when comparisons have been made between 

 them. The importance of certain physiological conditions 

 will be emphasized in the next section. The most signi- 

 ficant facts are that the methods do not in general agree, 

 and that the tests to which they have been subjected, in par- 

 ticular by Abney and by Tufts, were under conditions radically 

 different than those of practical photometry. The most 

 pressing subject for investigation just now is, therefore, the 

 relation between these two methods, under various conditions. 

 What the important conditions are will be made clear by 

 consideration of certain physiological factors. 



Physiological Factors, 



The relative sensibility of the eye to different coloured 

 lights is considerably affected by changes in illumination and 

 by changes in the size of the field of view. According to 

 present theory, these differences are due to the irregular 

 structure of the retina and to the preponderance of different 

 seeing elements at different illuminations. Two sets of 

 elements are present in the retina — rods and cones. Ac- 

 cording to the " Duplicitats Theorie " of von Kries, the rods 

 are chiefly responsible for vision at low intensities, the cones 

 at high. The cones are sensitive to colour with a maximum 

 of sensibility in the yellow ; the rods are not sensitive to 

 colour, but have a maximum of sensibility at a wave-length 

 corresponding to the green. These elements are distributed 

 unevenly over the retina. At the centre (the fovea) there 

 are no rods ; both rods and cones are about equally dis- 

 tributed in the surrounding central region called the macula 

 lutea; while in the outer regions of the retina rods pre- 

 dominate. Hence, either increasing the size of field or 

 decreasing the illumination brings into play more of the 

 rods, thereby explaining the Purkinje and yellow-spot 

 effects. 



Whether the rod and cone theory will prove the true 



