Solar spectrum, illustration.
Solar spectrum, illustration.
Infra red rays and the lower frequencies of the luminous spectrum in therapeutics / W.D. Chesney. Chicago : McIntosh Electrical Corp., [1920?]. C5246i 1920z Osler - Robertson
This figure by W. D. Chesney from around 1920 depicts the light spectrum. It is part of a technical guide to light therapeutics. Finsen had earlier discovered that the red light of the spectrum was the weakest in terms of concentrated chemical action. Red light is furthest on the spectrum from UV and violet light--the highest in chemical ("actinic") and bactericidal action. This is how Finsen was able to treat lupus with ultraviolet light.