At the same time the influence of violet rays on animal tissue is by no means invariably beneficial; they are often too powerful a stimulant. That the violet rays have an influence on the human skin which in the first place, at all events, is destructive and harmful in a high degree, is now clearly established by the observations and experiments of Charcot, Unna, Hammer, Bowles and others, while Finsen has made an important advance in the treatment of disease based on this fact. The conditions called ‘sun-burn,’ ‘snow-burn,’ ‘snow-blindness,’ for instance, which may affect even travelers on snow-fields and Arctic explorers, are now known to be wholly due to the violet and not to the red rays. Unna’s device of wearing a yellow veil, and Bowles’s plan of painting the skin brown, thus shutting off the violet rays, suffice to prevent sun-burn. The same effect is also obtained by nature, which under the stress of sunlight, and largely through the irritation of the violet rays themselves, weaves a pigmentary veil of yellow and brown on the skin, which thus protects from the further injurious influence of the violet rays and renders the sunlight a source of less alloyed joy and health. That the presence of the red rays, or at all events the exclusion of the violet, is of great benefit in many skin diseases seems to be now beyond doubt. This has been shown by Finsen in his treatment of smallpox in red rooms; it appears that it was also known in the Middle Ages as well as in Japan, Tonquin and Roumania, red bed-covers, curtains or carpets being used to obtain the effect. Under the treatment by red light not only is the skin enabled to heal healthfully without scarring, but the whole course of the disease is beneficially affected and abbreviated, the fever is diminished and also the risk of complications. Another physician has discovered that a similar beneficial effect is produced by red light in measles. A child with a severe attack of measles was put into a room with red blinds and a photographic lamp. The rash speedily disappeared and the fever subsided, the child complaining only of the absence of light; the blinds were consequently removed, and the fever, rash and prostration returned, to disappear again when the blinds were resumed.