72 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



SUMMARY. 



The essential features of our results may be summarized as follows : 



(1) In moving' across the retina, the images of colored objects of 

 moderate size and luminosity pass through certain regular series of 

 transitional tones. On the basis of these changes of color-tone we may 

 conceive the retinal surface to contain three concentric zones an ex- 

 treme peripheral zone upon which all stimuli of moderate area and 

 luminosity appear colorless ; an intermediate zone where they appear 

 yellow or blue (two exceptions will be mentioned later), and a central 

 zone where red and green appear. 



(2) The position and extent of the color zones are not fixed, but 

 variable. Their area in any given case depends upon the momentary 

 condition of retinal adaptation, and upon the brightness and saturation 

 of the stimuli employed. (Other investigators have established the 

 fact that zonal extension is also a function of the character of the back- 

 ground, the condition of optic refraction, and the magnitude of visual 

 angle of stimulus.) Hence the absolute and relative extension of the 

 zones may be varied at will. It is possible, for example, to choose a 

 violet stimulus which can be recognized as violet at the extreme peri- 

 phery, and it is equally possible to choose another stimulus of the same 

 tone of violet which can not be recognized as violet at the center of 

 the fovea. 



(3) Of all possible colors, there are four and only four which 

 undergo no change of tone in indirect vision. These are a purplish- 

 red (non-spectral), a yellow (about 570 /*/<), a bluish-green (about 

 490 /*/*), and a blue (about 460 /*/*) .* 



(4) When stimuli representing these four stable colors are equated 

 in white-value and in color-value, the retinal zone upon which the red 

 is recognized coincides with that of the green, and the zone of the 

 yellow coincides with that of the blue. The extension of the yellow- 

 blue zone is considerably wider than that of the red-green zone. 



(5) When a color stimulus is applied to an eccentric region of the 

 retina, the color rapidly fades out of the image. The fading process 

 follows a gradual course, and passes through a regular series of transi- 

 tions. Here, too, the order of change is through yellow (for stimuli of 

 long-waved tones) or blue (for stimuli of short-waved tones) to gray. 

 If the stimulation be sufficiently intensive or sufficiently long-continued, 



*These numbers represent the wave-lengths at the center of the spectral bands 

 transmitted by our stimuli. A more accurate description of 'the composition of 

 the stimuli will be found on p. 60. 



