It is now about eight years since Professor Finsen of Copenhagen published the results of his first investigations concerning the action of light upon the skin. The results of his experiments were essentially the same as those obtained by a number of other investigators, notably Widmark of Stockholm, namely, that sunburn or erythema solare is not caused by the heat of the sun, as was formerly believed, but is due to the action of the ultra-violet or so-called chemical rays.
Having convinced himself of the fact that the blue and the ultra-violet rays of light were capable of inciting an inflammation of a healthy unprotected skin, Finsen concluded that it would be of material benefit to exclude these rays in the treatment of certain inflammatory conditions of the skin, for example, variola; since in that disease suppuration and pitting are most marked upon the parts exposed to light. In summing
CLEMENSEN PC. A BRIEF REVIEW OF FINSEN'S PHOTOTHERAPY.. JAMA. 1902;XXXVIII(15):919-925. doi:10.1001/jama.1902.62480150013001a