In the following experiments it is shown that the exposure of rachitic rats to sunshine transmitted by common window glass outdoors markedly raises their resistance to an oral infection with "rat typhoid," although the rickets is not improved. The beneficial effect of exposure to fresh air alone is approximately half that of the combination of fresh air and sunshine through glass. The addition of vitamin D to this rachitogenic diet also increases the resistance of the rats to the infection, and if they are exposed to sunshine under window glass as well, the resistance is still further raised.
The radiation from the sun that reaches the earth possesses wavelengths that vary from 290 to 4,000 millimicrons1 (mμ), or from 2,900 to 40,000 angstrom units. These radiations are electromagnetic and comprise but a small portion of the entire known range of wavelengths, which vary all the way from