COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 69 



substance of the periphery. This view is advocated by Schb'n and by 

 Fick and is ultimately adopted by Helmholtz as representing his final 

 position. We have elsewhere (pp. 3Qf.) shown cause for refusing to 

 accept it. 



(d) According to the theory of Mrs. L/add-Franklin, the visual 

 substance is to be conceived as a product of a progressive development, 

 in which three stages may be differentiated. In its primitive form the 

 photo-chemical substance is decomposable by light of all sorts the 

 conscious product of the decomposition being an achromatic sensation. 

 In this form the substance occurs only in the rods, and this supposition 

 coincides with the fact that the extreme periphery of the retina con- 

 tains rods almost exclusively. In its second stage of development, where 

 it occurs only in the cones, the visual substance is differently decompos- 

 able by long-waved and by short-waved light. Here arise the sensa- 

 tions of yellow and of blue, besides the achromatic series. This is the 

 stage of development which is represented by the " dichromatic " color 

 zone of the normal retina, i. e., the zone in which, with moderate con- 

 ditions of stimulation, blue and yellow are the only colors perceived. In 

 its completely developed form the yellow-producing substance has 

 reached a still higher stage of development and is now capable of being 

 separately decomposed by erythrogenic (red-producing) and chloro- 

 genic (green-producing) rays of light. This form of the photo- 

 chemical substance occurs in considerable quantity only in the central 

 and paracentral regions of the retina (hence called by Mrs. Ladd- 

 Franklin, the " tetrachromatic zone"). The normal retina thus con- 

 tains visual substance in all three stages of development ; and the propor- 

 tion of the less highly developed substances increases with increase of 

 distance from the center. 



This theory has the advantage of being cast in an evolutionary 

 mold, and of accounting for the relative subjective imperfection of the 

 peripheral retina in evolutionary terms. It connects this imperfection 

 with the relatively undeveloped structure which is known to be char- 

 acteristic of the periphery, and with the fact, established by Ramon y 

 Cajal, that the rods are undeveloped cones. 



The hypotheses of this theory readily fall into line with all of the 

 facts revealed by the present investigation. The differently decompos- 

 able substances are so distributed as to render it plausible that the retina 

 should contain three concentric zones, and that these zones should vary 

 in extent with variable conditions of stimulation. The retinal process 

 which underlies the fading of peripherally seen colors through yellow 

 or blue to gray, is rendered easy of envisagement by the hypothesis 



