Understanding that the antira-chitic potency of sunlight and arti-ficial light was limited to a narrow band of ultraviolet radiation was the next major advance in the history of vitamin D3. Hess showed
that exposure to a mercury vapor lamp (spectral range=230–595nm) for 3 minutes from a distance of 3 feet prevented the onset of rickets in rats maintained on a standard diet.59 Interposing a
Corning window glass filter (2.6mm thickness) rendered the mercury vapor lamplight ineffective at preventing rickets.59 The filter blocked ultraviolet rays of less than 334 nm, suggesting that
these wavelengths were responsible for the antirachitic effect.59 Hess and Mildred Weinstock
further characterized the antira-chitic spectra of ultraviolet radia-tion.72 They studied the prevention of rickets in 4-ลweek-old rats on a rachitogenic diet by exposing