CH. LVI.] ELECTRICAL CHANGES IN RETINA 803 



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

 ditions 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 sensation ; then it pro- 

 duces 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 hs the " photo- chromatic interval." Red 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 bright- 

 est 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 colour- 

 less 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 

 is the same as in extra-foveal vision of the normal eye ; in many of these cases there 

 is a central scotoma (blind spot), that is, the rod-less fovea is blind ; there is reduced 

 acuity of vision as in the " dark-adapted " eye, and photophobia (fear of strong light); 

 nystagmus (oscillating movements of the eye) also occurs due to absence of an area of 

 distinct vision. We are thus in typical cases of achromatopsia dealing with cases 

 of cone blindness. In nyctalopia (night blindness) on the other hand, we meet the 

 converse condition. Here there is an abnormal slowness of "dark adaptation," and 

 a pathological change known as retinitis pigmentosa is present, suggesting an im- 

 paired function of the visual purple. Pilocarpine has been found an effective drug 

 in such cases, and this is also interesting because it hastens the regeneration of visual 

 purple in the extirpated eye. 



The electrical variations in the retina under the influence of light have been 

 recently reinvestigated by Waller. The excised eyeball of a frog is led off by non- 

 polarisable electrodes to a galvanometer. One electrode is placed on the front, the 

 other on the back of the eye. If the eyeball is quite fresh, a current is observed 

 passing through the eyeball from back to front. When light falls on the eye this 

 current is increased ; on shutting off the light there is a momentary further increase, 

 and then the current slowly returns back to its previous condition. Waller explains 

 this by supposing that anabolic changes in the eye predominate during stimulation 

 by light. With the onset of darkness, the katabolic changes cease at once, and the 

 anabolic more slowly ; hence a further positive variation. 



As already stated, the current in a fresh eyeball passes from back to front before 

 the stimulus is applied, but this cannot be regarded as a true current of rest, but as 

 a current due to previous action which very slowly subsides. When this has 

 subsided, the true current of rest is from cornea to fundus, '.#., it is like that of the 

 skin (see p. 47-3) ingoing the response to stimulation is like that of the skin out- 

 going. Waller has also studied the electrical responses of the eyeball to other 

 methods of stimulation ; if electrical currents are employed, and the eyeball is still 

 healthy, the response is always an outgoing current, whatever may be the direction 

 of the electrical current used as the stimulus. These currents of action are no doubt 

 mainly of retinal origin, but later Waller showed that the anterior portions of the 



