66 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



Hellpach's "gegenfarbige Zone" is doubtless a product of the 

 latent after-effects of stimulation (retinal fatigue), which we have 

 described (pp. 57ff.). It will be remembered that in determining the 

 position of the zonal limits, he moved his stimulus in from the periphery, 

 each exposure being made at a point ten degrees nearer the visual axis 

 than its predecessor. This procedure was continued until color was 

 perceived ; but since the steps had been too long to guarantee an accu- 

 rate determination of the limits of the color-zone, he went back 20 

 degrees from the point at which color was first reported and readvanced 

 over the same ground with shorter steps. Of course, this method pro- 

 vided the most favorable conditions possible for the operation of the 

 residual after-effects of stimulation. For his final determinations were 

 invariably made upon a region which he had previously fatigued. It 

 was inevitable that this method should yield the abnormal results which 

 he reports. And if he had followed the same procedure in his explora- 

 tion of the other regions of the retina, he would doubtless have found 

 that the whole retinal surface, save the fovea and the blind-spot alone, is 

 one vast ''gegenfarbige Zone." On the other hand, our own results 

 justify the prediction that if he had given the retina an adequate rest 

 before each stimulation, he would have found no trace of a "gegen- 

 farbige Zone " at any part of the retina. 



