THE PERCEPTION OF LIGHT AND COLOR. 191 



we are not obliged to assume that each kind of fibers constitutes a 

 bundle of which the base forms a continuous portion of the field of 

 view. To quote Durand: "On the contrary, we may conceive that 

 these three different kinds of fibers are intercombined in their simple 

 units in such a way that all parts of the retina present a homogeneous 

 mixture of these fibers, so that on whatever point of the retina a ray 

 of a given color should fall it would be sure to encounter there a 

 corresponding fiber adapted to receive its impression." 



We have just seen that Durand assumes that three kinds of fibers 

 are sufficient to produce all color sensations, and this is also the num- 

 ber adopted by Helmholtz, but the latter attributed to them the per- 

 ception of the red, green, and violet sensations, while Durand considered 

 the red, yellow, and blue sensations as primary. 



As it is hardly jiossible to assume that the light waves are trans- 

 mitted as such through the nerve fibers; the velocity of propagation of 

 nerve excitations being incomparably less than that of ether waves, it 

 is evident that one must assume that they undergo some transforma- 

 tion in the retina. This explains why the theory of Helmholtz is in 

 very good agreement with the hypothesis that there is an intervening 

 step of a chemical nature between the luminous radiation and the exci- 

 tation of the optic nerve. Koenig has remained faithful to this theory, 

 although he includes a fourth sensation, the gray sensation, due to 

 the decomposition of the visual purple. Experiments seem to him to 

 indicate that the retinal yellow is involved in the production of the 

 blue sensation; the red and green sensations he attributes to substances 

 still unknown. The inclusion of the gray sensation appears to me, 

 however, to necessitate the assumption of a fourth kind of nerve fiber, 

 and in addition there would still remain to be discovered two new 

 visual substances. 



Hering assumes that there are only three visual substances, but he 

 boldly attacks the dogma of specific nerve fibers, for he supposes that 

 the opposite chemical reactions [anabolic and catabolic changes], which 

 each of these three substances is capable of undergoing, produce the 

 complementary sensations (green, blue, and black, corresponding re- 

 spectively to red, yellow, and white). 



Ebbinghaus is said to have adopted this theory, completing it by 

 means of some recent discoveries; but it appears to us, on the con- 

 trary, that he has altered the fundamental idea, for he assumes that 

 each of the substances is capable of undergoing not two opposite 

 chemical reactions, but a single reaction iu two stages. The absorp- 

 tion spectrum of visual purple has its maximum between the D and E 

 lines of Frauenhofer; the yellow, resulting from the partial decomposi- 

 tion of the red, has its maximum decomposition between the F and Gr 

 lines. But the study of a subject affected with Daltonism, for whom 

 there are only two colors — yellow and blue— has shown that while the 

 region of the spectrum in which the blue is for him most brilliant coin- 



