To absorb as much of the active rays as possible, it is best to wear light or light blue or light gray clothing, which allows the sun’s rays to pass, whereas dark cloth does not, as found by Boubnoff and Lenkey. The rays of the sun are always valuable, but their action varies with the altitude. Thus, the higher the altitude and the rarer the atmosphere, the more efficacious will be the action of the sun’s rays. In lower altitudes the rays have to pass through dense strata of air filled with vapors of carbonic acid and dust, and thus much of their strength is lost. As Prof. Mohn[246] says in his book on “Meteorology”: “The rays of the sun in transit meet always denser and denser air, which contains large quantities of vapor, carbonic acid, and dust. A part of their strength is absorbed by the substances contained in the air, and these, as well as the air, are heated. Hereby some of the power of the sunshine gets lost, as the rays of the sun are reflected off these substances. Furthermore, they pass through clouds. Hence the rays of the sun lose more and more of their strength before they reach the earth.” Footnote 246: Mohn: Quoted after Holm, Norsh Magazin. Laege, W. 6, 1906. It is also an important fact that the higher the altitude, the more numerous are the chemical rays of the sun which have the greatest heating properties. In high localities sunshine contains much more of the blue and ultra-violet rays, whose wonderful action upon the red blood-corpuscles has been shown by the experiments of Niels Finsen.[247] Not only on the blood, but also on the nerves, they exercise a tonic action. As Niels Finsen has shown, it is due to these blue and violet rays that insects regain their vitality as soon as the sun shines. As Dr. Holm says, it is probably due to these rays that the quantity of red blood-corpuscles and of hæmoglobin is increased at altitudes of 500 or 600 meters above the sea level, as found by Viault[248] and Mintz.[249] Footnote 247: Meddelelser fra Finsen’s “Chemiske Lysinstitut,” Kjöbenhavn, 1899. Footnote 248: Viault: C. R. Acad. Sciences, p. 917, 1890 and p. 295, 1891. Footnote 249: Mintz: C. R. Acad. Sciences, p. 298, 1891.