38 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



that an error should recur at frequent intervals in face of repeated 

 demonstrations of its erroneousness, and that it should reappear in such 

 recent investigations as those of Kirschmann and of Hellpach, is 

 scarcely comprehensible. Yet an examination of the work of these two 

 writers shows that their experiments and discussions proceed through- 

 out upon the tacit assumption that the retinal extension of color sensi- 

 tivity is a function solely of the color tone of the stimulus ; and that 

 differences in the brightness and saturation of the stimuli employed can 

 have no influence upon the degree of eccentricity at which the colors 

 of the stimuli are recognized. 



This fundamental objection may be urged against all of the earlier 

 and nearly all of the later investigations of our second problem. In no 

 investigation with spectral colors, and in only four investigations with 

 pigment colors, have the objective conditions of experimentation been 

 so arranged as to provide for the variation of but a single factor at a 

 time. However valuable, then, these other experiments may be in 

 determining the number and variety of color changes which occur in 

 indirect vision, they are utterly incompetent to warrant any inference 

 whatever as to the relative areas of the different color zones. And 

 indeed of the four papers by Bull, Hess, Hegg, and Guillery which 

 remain when the culling process has been completed, that of Guillery 

 is found to have met the conditions imposed only approximately. 



"A survey of the papers which remain shows that all three agree in 

 finding the following result : The retinal sensitivity to a certain red is 

 co-extensive with that to a certain green; the sensitivity to a certain 

 yellow is co-extensive with that to a certain blue; the former pair of 

 color zones is much narrower than the latter pair. 



Several fundamentally different conceptions have been advanced to 

 explain the facts of indirect vision. In most instances these conceptions 

 have been wrought out in an attempt to reconcile the facts with color 

 theories already in the field. Since all of the present theories of color 

 vision are the product of a comparatively recent period, and since, more- 

 over, some of them have been modified in greater or less degree since 

 their initial formulation, it was inevitable that in the history of our 

 problem different interpretations should be put upon the phenomena in 

 question. 



Aubert found that the phenomena of indirect vision are analogous 

 throughout with those of direct vision. He also discovered that the 

 peripheral retina is much more readily fatigued than the central region, 



