﻿° BOTANICAL GAZETTE [january 



journals in England, America, Germany, and France (3, 4, 5, 6, 7). 

 The experimental basis of Finsen's phototherapy is (1) the bac- 

 tericidal property of the chemical light waves; (2) the power of the 

 chemical rays to produce erythema; (3) the power of the chemical 

 rays to penetrate the skin. The bactericidal property of the chemi- 

 cal rays had been demonstrated by previous workers. Widmark 

 had proved by experiment the fact, pointed out by Charcot 30 

 years before, that photoerythema is produced by the chemical rays 

 of light and not by the heat rays. Goodneff and Finsen demon- 

 strated the powers of chemical rays to penetrate the skin, by placing 

 sealed glass tubes containing silver chloride under the skin of cats 

 and of dogs and exposing to light. The silver chloride was black- 

 ened. Finsen also showed that light will penetrate bloodless 

 tissue, but will not penetrate tissue containing blood. He placed 

 strips of sensitive paper on one side of a man's ear and aUowed blue 

 and violet rays of concentrated sunlight to fall upon the other side 

 of the ear. After 5 minutes the sensitive paper was not affected, 

 but if the blood was forced out by pressing the ear between glass 

 plates, the paper was blackened in 20 seconds. In agreement with 

 this is the fact that the spectrum obtained by passing light through 

 an ear filled with blood consists of only a red stripe, while the spec- 

 trum obtained by passing light through an ear made anemic con- 

 sists of all colors. 



It is worth while to consider in some detail the method used 

 in the practice of Finsen's phototherapy. A carbon arc, carrying 

 50-60 amperes, is used as a source of light. Previous to 1901 sun- 

 light which had passed through a concentration apparatus was used 

 for treating the patients. Its use was abandoned because, aside from 

 the uncertainty of weather conditions, it was found that sunlight 

 is not only weak in the extreme ultra-violet region (the thera- 

 peutically effective part of the spectrum), but it contains an abun- 

 dance of light in the blue-violet region. The blue-violet waves so 

 tan the skin that after one or two treatments the deposit of pigment 

 makes further treatment impossible. The carbon arc, on the other 

 hand, emits light of shorter wave lengths than those found in sun- 

 light. These short light waves have a marked action upon the 

 surface layers of the skin. They destroy many of the epidermal 



