SECTION II 



THE DUPLICITY THEORY 



The duplicity theory (v. Kries) states that achromatic scotopic 

 vision is carried out through the mediation of the rods alone, the 

 cones being the organ of photopic vision. It is not definitely stated 

 that rod activity is in abeyance in photopic vision, but there is some 

 evidence in favour of this view and in any case the influence of rod 

 vision in the light-adapted eye may be regarded as slight. 



It has already been necessary for the sake of clearness to point out 

 the great probability of two separate mechanisms being involved in 

 these two very different types of vision {v. p. 57). Broadly speaking 

 vision with the dark-adapted eye, i.e., scotopic vision, is monochromatic 

 or tone-free. Vision with the light-adapted eye, i.e., photopic vision, 

 is polychromatic or toned. In the former the threshold stimulus 

 intensity is low ; in the latter relatively high. We have hitherto 

 stated the facts and attempted to regard them impartially. It will 

 now be instructive to review them briefly in the light of the duplicity 

 theory. 



So far as anatomical considerations are concerned the distribution 

 of rods and cones in the retina is of prime importance. Rods are absent 

 from the fovea, an area subtending a visual angle of 50' — 70', and 

 also over a surrounding zone, the whole subtending a maximum of 3°. 

 At the same time it should be pointed out that the structural peculi- 

 arities of the rods and cones are not precisely differentiated. The 

 macular cones are elongated and more rod-like than those found in 

 peripheral parts of the retina^. This merging of structural peculiarities 

 is even more marked in the retinae of lower animals^, and while on the 

 whole later researches have confirmed the observations of Max Schultze 

 {v. p. 196), precise anatomical diversity of the two types of neuro- 

 epithelium has been somewhat discounted. Moreover, if we regard the 



1 Cf. Greeff, in Graefe-Saemisch, 2nd ed. Bd. i. Abt. 2, Kap. V. 



- Cf. Putter, in Graefe-Saemisch, Bd. n. Abt. 1, Kap. x ; Franz, in Oppel's Lehrbuch d. 

 vergleich. mikr. Anat. d. Wirbeltiere, vn. Jena, 1913. 



