COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 37 



therefore take into account the influence of all of these co-operating 

 factors, since, too, it turns out that they are non-independent variables, 

 and that they are, for that reason, difficult to control, he finds himself 

 called upon to exercise the highest degree of foresight and caution. 

 Several investigators have, indeed, attempted to solve the difficulty by 

 cutting the Gordian knot. Failing to appreciate the magnitude of their 

 task, they have ignored the existence of certain of these essential con- 

 ditions ; and they have employed methods which invited disaster from 

 the outset, simply because no provision was made for the control of the 

 determinants of the values which it was hoped to establish. Needless 

 to state, these experiments have been barren of result, so far as the solu- 

 tion of the present problem is concerned. The influence of unequal 

 brightness seems to have been the chief obstacle in the path of this type 

 of investigator. It has been established in hosts of instances that change 

 of luminosity of stimulus is, within limits, invariably attended by a 

 corresponding change in the extension of the retinal zone within which 

 the color of the stimulus is recognized. Not only has this fact been re- 

 established at frequent intervals during the past fifty years, but its 

 validity has never been disputed by any investigator whose stimuli cov- 

 ered an adequate range of luminosities. Its significance for the prob- 

 lem is self-evident. No determination of the relative extensions of the 

 various color zones can ever yield really comparative results unless it 

 be accomplished by means of stimuli of equal brightness, or, more cor- 

 rectly, of equal white-value. Nothing could be clearer. Yet only a 

 very few investigators have appreciated the significance of this fact. 



The conditions with which one has to deal here are analogous with 

 those which exist in the domain of audition. An illustration borrowed 

 from acoustics may therefore serve to emphasize our point. It is a 

 familiar fact that the range of audibility of tones is a function of their 

 intensity. Now, let us suppose that we wish to discover whether the 

 carrying power of tonal stimuli is not also a function of their pitch. 

 And let us suppose further that for the stimuli to be employed in our 

 investigation, we choose at random a series of tones whose intensities 

 we neither know nor care to determine. Is it not clear that our results 

 would be valueless, and that they would warrant no inference whatever 

 as to the relation which we set out to investigate? 



Yet an equally haphazard procedure has been followed in many of 

 the attempts which have been made to determine the relative range of 

 visibility of a series of color tones. That the pioneers in this field 

 should, in their generation, have failed to appreciate the significance of 

 one or other of the factors under discussion, is not surprising. But 



