The chemical action of sunlight is exercised by the blue and ultra-violet rays (Finsen), and the heat comes mainly from the red rays. From our point of view, however, the chemical rays play the more important part. When we sit in the sun for a long time and get an inflamed skin, this is due mainly to the chemical rays. This fact was established by Charcot on the basis of clinical observations. Charcot’s work has been confirmed by the researches of Widmark, and especially by the exact experiments of the famous Danish physician, Niels Finsen,[243] in 1906. The latter showed that when strong light killed bacteria, this was due to the action of the chemical rays alone, and, specifically, to the ultra-violet rays. He also showed that these are the rays which produce dilatation of the blood-vessels and an inflammatory condition of the skin. Footnote 243: Finsen: “Om anvendelsen af concentrerede chemiske Lysstraaler,” Kjöbenhavn, 1896. To the red cheeks of those people who pass much time in the open air and sun, we can compare, as a contrast, the pale faces with a greenish hue of those who live in the dark, like polar explorers. If strong light dilates the blood-vessels, and sends much blood to the periphery of the body, thus promoting the insensible perspiration and metabolism, on the other hand, in the long night of the arctic regions there is contraction of the blood-vessels, and the blood is kept back in the interior of the body, with the retention of excretory products. It has been shown by Oerum,[244] through experiments on animals, that the quantity of their blood and its percentage of hæmoglobin is dependent upon the light. They are reduced in the darkness and increased in the light. Through a light bath the quantity of blood was increased 25 per cent. within four hours. Finsen[245] has also found, through examination of twenty-nine persons, that there is less hæmoglobin in the blood in winter than in summer, due to the lack of sunlight in winter. Grawitz and Graffenberger have seen a diminution of the hæmoglobin, as also of the quantity of the whole blood, in animals which were kept in the dark. Marti found that the red blood corpuscles are diminished in such animals, but become augmented when they are again exposed for a time to the sunshine. Footnote 244: Oerum: Pflüger’s Archiv. f. d. g. Physiologie, vol. cxiv. Footnote 245: Hospitalstidende, p. 1209 and 1239, 1894. We should aim to get our share of direct sunlight in the open air, for, as Finsen has shown, the valuable chemical rays of the sun are excluded by glass; and, after all, it is not warmth alone we seek, but also the chemical and anti-bacterial action of the sunlight, together with its effects upon the blood-vessels and nervous system.