522 PRINCIPLES OF GENERAL PHYSIOLOGY 



used. Now Victor Henri et Larguier des Bancels (1911, 1) have determined the 

 amount of energy just sufficient to excite (threshold energy), the bleaching effect 

 on the pigment and the amount of light absorbed by it, all at various wave 

 lengths between the values named. When put into curves, these three factors 

 are found to follow the same course (see Fig. 164). This means that to produce 

 the same sensation, by different wave lengths, requires such an amount of radiant 

 energy that the amount absorbed by the visual purple is the same in all cases. 

 This is a powerful argument in favoup of the participation of the pigment in 

 vision, at all events in that particular form of the sensation investigated. The 

 same investigators find that the absolute quantity of energy required varies with 

 the duration of action according to a complex law, which seems to result from a com- 

 bination of that of excitation of nerve with that of a photo-chemical reaction. If 

 the energy quantum be worked out by the formula in Nernst (1913, 255, etc.), it 

 is 2 x 10" 1J erg for the D line, practically the same as the limit of the sensibility 

 of the retina (page 512 above). This sensibility is then the maximum possible. 



Electrical Changes. Holmgren (1880) noticed that the incidence of light is 

 accompanied by an electrical change in the retina, and further work was done 

 by Dewar and M'Kendrick (1876), Kuhne and Steiner (1880), Waller (1900), 

 Gotch (1903 and 1904), Einthoven and Jolly (1908), Piper (1905, 1910, and 1911), 

 Frohlich (1913), and others. Although, when the interpretation of these results 

 is better understood, they will undoubtedly help in the explanation of retinal 

 processes, it must be confessed that, at present, they do not throw much light 

 on the question. The main fact is that, in the uninjured eye of the vertebrate, 

 the incidence of light causes an electrical change in such a direction that the 

 nervous layer of the retina becomes electrically positive to the rod and cone 

 layer. This state, with some subsidiary waves, lasts during the illumination, and 

 disappears again, at a certain rate, when the light is removed. But, immediately 

 after removal of the illumination, and before the effect produced by it has 

 disappeared, a puzzling further change, in the same direction as that produced by 

 the incidence of light, makes its appearance for a short time. Apparently darkness 

 causes a temporary effect of the same sign as light does. When, however, the 

 curve is carefully analysed, as can be done with that obtained by the capillary 

 electrometer or the string galvanometer, it is found to have a complex form, which 

 Einthoven and Jolly and Piper have resolved into a compound of three different 

 curves of different time course, and it is important to note that, by proper con- 

 struction of each of these components, it is possible to obtain a curve like the 

 original, including the " dark " effect, from curves which have an opposite direction 

 in light and in darkness. This fact disposes, indeed, of some of the difficulty 

 attending the dark effect, but seems somewhat artificial, since no explanation can 

 yet be given of the meaning of the three hypothetical processes. It is true that the 

 Young-Helmholtz theory supposes that there are three fundamental components in 

 visual sensations, red, green, and violet. When in certain relative proportion, the 

 sensation of white light results and that of various colours according to the 

 relative proportion of the three components. But Gotch's experiments (1904) 

 showed that the electrical response to red, green, or violet light was of the same 

 form in each case, although differing in latent period and magnitude, while each 

 showed a similar rise when the stimulus was removed. The three hypothetical 

 components of the curve, assumed by Einthoven and Jolly and by Piper have, 

 as it seems, nothing to do with the three fundamental sensations of the Young- 

 Helmholtz theory. We shall see other reasons later why this theory cannot be 

 regarded as a satisfactory one. Still less do the facts of the electrical change 

 support theories such as that of Hering, where colours which are complementary to 

 one another, that is, which make white on mixing, are supposed to cause opposite 

 changes in the " visual substances," the one anabolic, the other catabolic. If this 

 were so, red and violet should give electrical changes of opposite sign. 



As examples of actual experiments, Fig. 165 (according to Piper. 1910) may 

 be of interest. The electrical change in the mammal is of a simpler nature than 

 that in the bird. Piper also shows (1911, Figs. 38 and 39) that the response i- 

 a simple one in the retina of the Cephalopod, a fact which indicates that a part 



