ditions to the movements and displacements of retinal pigment grains in course of chemical decomposition.
There is another visual phenomenon caused by mescal, to which my attention was particularly attracted, since it appears to have been little noticed by previous observers, and which is of considerable interest because it may be brought into line with various phenomena which occur without the intervention of drugs. I refer to the play of shadowy color, and more especially the violet halos which are seen to play around and over objects and constitute, in my own case at all events, the earliest group of color phenomena seen under mescal. I have already described my impressions of this effect, and one of my subjects who saw the same phenomenon, describes it as 'a violet fringe, surrounding objects and tending to become flower-like.' It has been observed by many when passing over snow-covered regions, and especially by Alpine climbers, that moving objects, and more especially their own hands or garments, show a violet border. Then we have the tendency to color vision (erythropsia or perhaps more strictly, or more usually, violet vision) to which the eye becomes liable after surgical removal of the crystalline lens. Once more there are the colors produced by the much discussed colorless 'spectrum top.' It seems to me that all these phenomena, and others that could be named, must be regarded as more or less allied. The first explanation offered for the earliest of them to be noted was that they are due to over-stimulation and exhaustion of the eye. Later inquirers have sought after a more precise mechanism for the phenomenon. Thus Dobrowolski in 1887, dealing with the erythropsia often occurring after removal of the lens, argues that a necessary condition is the dilatation of pupil produced by atropine, and that the color vision is really of the nature of a negative after-image of the rays that strike the eye. Fuchs, in 1896, who has dealt in an interesting manner with the erythropsia experienced on climbing snow-covered mountains (regarded by him as strictly a purple vision) finds that Dobrowolski's explanation is inadequate, and, while contenting himself with the theory of stimulation and fatigue for some of the phenomena, believes that the real explanation is to be found in a temporary visibility of the visual purple of the retina. Whatever the value of this explanation may be as applied to the whole group of phenomena, Fuchs more than any one helped to bring color-visions of this kind from the sphere of the pathological into the sphere of the normal. More recently still, Shelford Bidwell in 1897, when making some simple but ingenious experiments with the object of discovering
the mechanism of the spectrum top, found that when a dark patch is
- This phenomenon was discussed in Nature, during May, 1897, some of those who described it assuming without question that it was an objective phenomenon.