SiR,-The question of the meaning of the structure of the ( retina is the latest one in physiological optics--i.e., from the subjectival standpoint held by E. Hering, Mach, and myself. As Edridge-Green has found for the distribution of the visual purple, so I, Hering, Hess, Garten, and others, have found only gradual quantitative differences in the sight between the foveal and extrafoveal area. The phenomenon of Purkinje, the alteration of optical white equations by the state of light-and

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... aptation, the colourless interval for spectral lights of increasing intensity, the different phases of the afterimage,-all these subjective reactions exist, not only in the extrafoveal, but also (only gradually diminished) in the foveal region. The analogy between this behaviour and Edridge-Green's objective statement about the visual purple is a striking one. As prinaipia, I believe that it is absolutely necessary for the classification of colours to start with sensation analyses, from the statement of the simple effects by some definite but individually different lights, homogeneous or mixed. It is also necessary to note the changes produced in each case by varying states of light-and dark-adaptation. We must ascertain the position of pure green, pure yellow, pure blue in the spectrum, and of pure red in mixed light, and of the corresponding neutral points in the colour-blind. It is totally wrong to conclude from the constitution of physical stimuli (composition, wave-length) that the physiological effect or the psychological sensation are simiJarly constituted. * Therefore all colour theories based on light mixture (colour mixture is a very bad expression), as the theory of Yonng-Maxwell-Helmholtz, A. Fick, J. v. Kries, are fallacious. Hering holds that for the regular cases of colour blindness there is a defect of red-green or yellow-blue or red-yellowgreen-blue perception. The antagonistic correlation between red and green and yellow and blue is founded in the nervous apparatus ; the regular cases of colour blindness are therefore nervous anomalies. On the other side, I believe that alone or combined with a nervous anomaly there are many cases in which photo-chemical anomalies in the retina exist.