364 PHYSIOLOGY CHAP. 



than in mammals ; and finally, that in some species of reptiles that 

 prefer bright light or the direct rays of the sun, as snakes, lizards, 

 and tortoises, the retina consists almost or exclusively of cones. 



Haab and Kiihne held the same view, and also based it on the 

 comparative predominance of rods and cones in the retina. 



The observations of Parinaud (1884) and still more of v. Kries 

 (1894) gave a wider basis to Schultze's theory. Von Kries assumed 

 that the rods are the receptors in scotojnc or twilight vision, and 

 the cones, particularly those of the fovea, the organ of photonic 

 or daylight vision. The characteristic differences in the physio- 

 logical properties of the two systems form according to v. Kries 

 the physical basis of the duplicity theory. 



The characteristics of the scotopic system are : 



() Total blindness to colours, whatever the wave-lengths of 

 the rays acting as stimuli, even when the retina is perfectly 

 adapted to darkness. 



(&) Preponderating sensitiveness to rays of medium or short 

 wave-lengths ; so that in the dispersion spectrum obtained through 

 a prism the maximal brightness lies between green and blue, 

 while red is quite dim. 



(e) Eetinal adaptation to darkness and consequent increase 

 of sensibility to light, which preponderates in the peripheral as 

 compared with the central portions of the retina. 



(d) Capacity of the outer segments of the rods to form visual 

 purple or rhodopsin, which, as it accumulates in the dark, is 

 probably the sensitising substance. 



On the other hand, the characteristics of the photopic system 

 for daylight vision, which the cones subserve, are : 



(a) Appreciation of chromatic sensations, when stimulated by 

 monochromatic rays of medium intensity. 



(&) Appreciation of white, when stimulated by a mixture of 

 light-rays, or by monochromatic rays of excessive or too low 

 intensity. 



(c) Visual acuity, which is maximal at the fovea, where only 

 cones are present. 



Certain objections have been raised to the theory of v. Kries, 

 but they do not appear to invalidate the facts. 



We have seen that the increase of retinal sensitiveness in the 

 dark-adapted eye does not depend entirely on the rods, for when 

 the fovea has been some time in the dark, it also exhibits a slight 

 increase of sensibility to light. But this only proves, as v. Kries 

 himself admitted, that adaptation does not depend exclusively on 

 accumulation of erythropsin in the rods, which augment the 

 sensitiveness of the entire retina, other factors being concerned. 

 Doniselli regards it as probable that the slight increase of 

 sensibility at the fovea is due to the marked increase of sensitive- 

 ness in the organ constituted by the rods. 



