1904-5.] NIE”S R. FINSEN—His LIFE AND WORK. 115 
already used light for the treatment of lupus, but it was either the heat 
rays of the focussing glass, or if light rays were used they were in too weak 
form to be of value. And so he determined to take up the s udy of this 
question from the beginning, as he seems to have done with every subject 
he investigated. And, as this germ-killing action of light is slow, it is 
necessary to concentrate it by mirrors or lenses, taking care to exclude 
the heat rays, the ultra-red, red, orange, and yellow, wh’ch would destroy 
the tissue by combustion, while the more easily diverted rays above them 
are the more active germ-killers. 
Be ore constructing any apparatus to put to practical use his many 
discoveries, he first made sure that the germ-killing action of light actually 
did increase ‘n proportion as the rays were concentrated. To do this, he 
coated the insides of two flat bottles with gelatine peptone, a material 
used for the cultivation of germs, and this soil he sowed with cultures of 
germs in pure bouillon (Bacillus prodigiosus which causes the red mould 
on bread, and Anthrax bacillus). Outside the bottles he gummed paper 
black on one side, white on the other; the white side out to avoid absorp- 
tion of heat rays, the black side next the glass to prevent the light affect- 
ing the cultures where so protected from it. Several round openings were 
cut in the papers, and across the openings were traced on the glass in 
Indian ink numbers, indicating in minutes how long each opening was 
exposed to light. From one to two hours after sowing the germs, the 
flasks were exposed at the same time, one to direct sunlight, the other to 
concentrated sunlight. They were then kept from one to two days in 
darkness to allow the cultures to develop, in which time the results were 
very apparent, for ‘‘the numbers indicating the space of time in which 
the light had killed the bacilli were clearly marked on the culture by the 
colonies which had developed in the shelter of the parts colored black. 
In this manner the bacteria themselves indicated the time of exposure 
necessary to kill them.’”’ By many similar experiments he proved that 
sunlight concentrated by means of his apparatus killed microbes fifteen 
times more rapidly than direct light, and that the effects of the concen- 
trated arc-light are very much more intense. 
As light penetrates all tissues, even the bones, and as oxygen is neces- 
sary to enable light to kill germs, and as blood contains more oxygen 
than any other constituent of the body, Finsen first thought that the more 
blood there was in the part to be treated by light, the better it would be. 
But he soon found out his mistake and tells us of it. Placing a piece of 
photographic paper (papier ariosto) behind the lobe of his wife’s ear, he 
allowed a cone of blue-violet light from his solar apparatus to fall upon 
the opposite side of the ear. At the end of five minutes there was no 
change in the paper, but on repeating the experiment while pressing the 
