LIGHT AS A BACTERICIDE. 
765 
As negative form of phototherapy must be mentioned, that 
made known by a Danish physician, Niels Finsen, some three 
years ago, who thought to treat variola with red light. Red 
light is one of the elements which cooperate in the formation of 
white or colorless ordinary lig-ht. Ret a sun ray fall and pass 
through a prism and it will appear enlarged, divergent and 
divided in monochromons rays, which are the colors of the 
spectra: purple, indigo, bine, green, yellow, orange and red. 
Red light is then ordinary light free from the other colors ex¬ 
cept the red. It is obtained from the spectra. And it is in that 
way that it is obtained for experiments in physics. But it is 
also obtained, with less purity, in passing ordinary light through 
red glasses, red stuffs, etc. Mr. Finsen has, then, utilized the 
red rays in the treatment of variola. 
Why these rays rather than others ? 
Because they have less chemical action, as it is known by 
photographers. Red rays have no specific action towards variola^ 
but they constitute a light where, with purely luminous rays, 
there is but a very weak proportion of chemical rays, which are 
most numerous in the purple extremity of the spectra and even 
extend beyond it in the ultra-purple. Chemical rays are irri~ 
tating and their irritation interferes with the healing of the 
eruptive buttons of variola. They can be eliminated either in 
the suppression of all light or the removing from the light of 
the part which contains them. 
The suppression of all light has often been utilized and his¬ 
tory records that the mother of St. Catherine of Sienna nursed 
her daughter in a dark room “ where no draught of air or a ray 
of light could penetrate,” so as to preserve her beauty against 
the marks of variola. In our days, without going so far as 
complete obscurity, attempts are made to obtain similar effects 
by covering the face with masks or opaque substances. But 
often the results are not considered as very satisfactory. 
As to the filtration of the light by substances which arrest 
the rays with irritating chemical action, those which, theoreti¬ 
cally, are injurious, it is obtained in the method of Finsen by 
