THE DUPLICITY THEORY 205 



discussed, and though in photopic vision they afford little incisive 

 evidence in favour of the duplicity theory they cannot be regarded as 

 seriously discounting it. It is certain that the almost, if not complete, 

 colour blindness of the extreme periphery cannot be attributed to mere 

 paucity of cones. The great difference in luminosity values with different 

 lights, in spite of the tone-free perceptions to which they give rise, is 

 against such a view. We must therefore conclude that the scanty cones 

 of the periphery are incapable of arousing colour perceptions, or that 

 there is some block in the conduction of such impulses from this region. 



The duplicity theory depends therefore for its support chiefly 

 upon the facts of achromatic scotopic in relation to those of photopic 

 vision, and in this relationship the evidence is abundant and confir- 

 matory. 



On this theory the rods are the end-organ of achromatic scotopic, 

 the cones of photopic vision. In other words, the rods possess a very 

 high degree of adaptation, the cones little or none. The photopic 

 apparatus is isolated in the fovea, but the scotopic apparatus is nowhere 

 completely isolated. The threshold values for the cones, however, are 

 considerably higher than for the rods, and hence in complete dark 

 adaptation the rods become almost, if not wholly, isolated physiologically. 

 In dark adaptation, with low intensities of light, vision is carried out 

 through the rods alone (achromatic scotopia). As the intensities are 

 increased the liminal stimulation values for the cones are exceeded and 

 vision is carried out by both rods and cones (chromatic scotopia). At 

 still higher intensities vision is carried out chiefly or wholly by the 

 cones (photopia), but whether the rod effects are added to the cone 

 effects or are abolished is as yet uncertain. 



If the rods are the organ of achromatic scotopic vision, the visual 

 purple, which so far as has yet been proved is present in them only, 

 attains a new significance. The early researches of Kiihne and Sewall^ 

 sufficed to show the slight effect of rays of long wave-length upon this 

 substance, and they found that the maximum efl'ect is in the green, 

 not in the part of the spectrum which is brightest to the photopic eye. 



Kottgen and Abelsdorff^ in more recent researches showed that the 

 curve of absorption values of the visual purple of mammals, birds and 

 amphibia with the interference spectrum of the Auer light has its 

 maximum at about 500 /x^u, of that of fishes at about 540 /x/x. These 

 results do not agree fully with Kiihne's, and further researches on human 



1 Untersuchungen, Heidelberg, ni. 221, 1880. 



2 Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xii. 161, 1896. 



