848 THE EYE AND VISION [CH. LV1II. 



the retina has become adapted to light of low intensity, the colours 

 of the spectrum alter in brightness ; the red end becomes shortened 

 and much darker ; the blue end becomes brighter, and the region of 

 maximum brightness is in the green. This change of brightness 

 with change of adaptation, known as Purkinge's phenomenon, is 

 absent in the fovea, where there are no rods. The selective action 

 of the colours of the spectrum on the visual purple is so strikingly 

 similar to the altered conditions of brightness just described, that 

 changes in the visual purple of the rods have been supposed to be 

 the cause of sensations excited by feeble illumination (i.e. in the 

 "dark-adapted" eye), while the cones are affected under more 

 ordinary conditions of illumination. This conclusion gains support 

 from several interesting facts. Visual purple is specially abundant 

 in the retinae of almost all animals whose habits are nocturnal, or 

 who live underground. Further, if the intensity of a colour stimulus 

 is gradually increased, it at first is too faint to produce any sensa- 

 tion ; then it produces a sensation of greyness, and at last the colour 

 itself is seen ; the interval between the appearance of the grey or 

 white-black effect and of the true colour effect of the stimulus is 

 spoken of as the "photo-chromatic interval." Eed light has no effect 

 on visual purple, and has no photo-chromatic interval (that is, it 

 appears either red or nothing), and according to several observers, 

 there is no such interval at the fovea, where the rods, and therefore 

 visual purple, are absent. Thirdly, a very similar effect has been 

 described by M'Dougall, when the retina is momentarily stimulated 

 by a coloured light ; the sensation arising from the stimulus is 

 followed by a series of " primary responses " or after-sensations ; the 

 first members of the series have the same colour as the stimulus, 

 and these are sometimes followed by a series of colourless (grey) 

 sensations ; these grey sensations are only present outside the fovea, 

 and under conditions of " dark adaptation " are absent with red and 

 brightest with green stimuli. Here again we are able to differentiate 

 between a visual-purple (rod) effect, and a cone effect, the former, 

 active under conditions of feeble illumination, affected most by green 

 and unaffected by red light, and yielding colourless sensations ; the 

 latter being more specially concerned in developing sensations of 

 colour under conditions of adaptation to ordinary light. The fovea 

 centralis thus becomes the region where the colours of objects are 

 best distinguishable, and where with ordinary illumination visual 

 acuity is most marked. In the dark, however, extra-foveal (rod) 

 vision is more sensitive than foveal (cone) vision ; astronomers see 

 faint stars more readily in the periphery of the field of vision. 



Two abnormal conditions may be described here, for they throw light on these 

 phenomena. In cases of achromatopsia (total colour-blindness) the spectrum is seen 

 as a band of light differing only in brightness ; the region of maximum brightness 



