THE MACULA LUTEA AND FOVEA CENTRALIS 81 



CHAPTER II 



THE MACULA LUTEA AND FOVEA CENTRALIS 



The Macula Lutea. The chief feature of macular vision is the 

 unequal absorption of different rays by the pigment of the yellow spot. 

 It has been necessary to refer to this disturbing feature at an earlier 

 stage (pp. 40, 70). 



The Fovea Centralis. The photopic luminosity curve for the fovea 

 is shown in Fig, 11, and calculations show that it is about one-sixth 

 more sensitive to the D light than the macula (Abney), To the green 

 and blue it appears to be less sensitive, and Konig^ even says it 

 is blue-blind. This observation is certainly untrue and is probably 

 due to the low scotopic luminosity value of the fovea combined with 

 the absorption by the pigment which probably extends to this region. 



We have already referred incidentally to many features in which 

 foveal vision differs from paracentral and peripheral vision. With its 

 exceedingly high sensibility for form we have little to do here, but it 

 should be borne in mind. More important for the present purpose are 

 its high photopic chromatic sensibility, and its very low scotopic sensi- 

 bility. 



It has already been mentioned that for white light achromatic 

 scotopic sensibility is proportional to the area stimulated (p. 52 ; 

 see also Section VI, Chap. i). For the fovea, in order to produce 

 a luminous sensation, the total quantity of light, i.e., the product of 

 the area and the illumination, must attain a certain value, and this 

 value is constant for a given condition of adaptation. 



Since the colourless interval increases in dark adaptation owing to 

 lowering of the general threshold, the chromatic threshold remaining 

 almost or quite constant {v. p. 60), and since dark adaptation increases 

 the foveal sensibility little or not at all, one might expect a very great 

 diminution or even absence of the photochromatic interval at the fovea. 

 Charpentier^ demonstrated a photochromatic interval for colours at the 

 fovea, and Koster^ and Tschermak^ confirmed the results, thus differing 

 from A. E. Fick^, Parinaud^, and Konig and v. Kries. If it exists for 



1 Konig, p. 353. ^ ^rch. cCOpht. xvi. 337, 1896 ; La Lumiere d /(.*■ Couleur«, p. 2U(i. 



3 Arch. J. Ophth. XLi. 4, 13, 1895. 



^ Arch.f. d. ges. Physiol, lxx. 320. 1898. =^ Ibid, xuii., 48L 1888. 



« Ann. d'ocul. cxii. 228, 1894 ; Arch. d'Opht. xvi. 87, 1896. 



P. 0. V. 6 



