MESCAL: A STUDY OF A DIVINE PLANT. 67 



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 espe- 

 cially by Alpine climbers, that moving objects, and more espe- 

 cially 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. 



