The effect of light upon the development of the smallpox eruption is a subject of considerable interest, and in recent years it has become one of therapeutic importance. As long ago as the fourteenth century John of Gaddesden and other physicians of his time were in the habit of excluding both light and fresh air from smallpox patients. The walls and furniture of the sick-room were painted red, on account of a peculiar virtue supposed to reside in this color, and the unfortunate occupant was nearly smothered by red curtains hung around his bed. Ever since that time it has been a common custom to darken the room of a smallpox patient, partly on account of the photophobia present during the course of the disease and partly on account of the idea that sunlight would aggravate the eruption. The fact that the face and hands are most intensely affected would seem to substantiate this idea, but the argument fails when we consider that the feet are usually the seat of an eruption scarcely less profuse. It was claimed by Black, in 1867, that the complete exclusion of light from the eruption of smallpox, even when occurring in unvaccinated persons, effectually prevented pitting of the face. Barlow, Gallivardin, and others, have expressed a similar belief. Experimentation by Finsen, Unna, and others having demonstrated that it was not the heat of the sun but the ultra-violet or chemical rays which cause solar eczema and pigmentation of the skin, it was suggested by Finsen that in place of the complete exclusion of light in the treatment of variola, it was only necessary to eliminate the chemical rays of sunlight by means of red glass windows or red curtains. Acting upon this suggestion Lindholm, Svensen, Day, and others, treated smallpox by this new method, and made most favorable reports of their results. The red light proved agreeable and soothing to the eyes of the patients, frequently caused the vesicles to dry without becoming purulent, and lessened the suppurative fever. The patients, it is claimed, passed directly from the vesicular stage into convalescence, and neither pitting nor pigmentation of the skin was observed. Some less enthusiastic experimenters with the red-light treatment of variola have been more moderate in their praises, and in some smallpox hospitals it has been tried and given up.