That seasonal variation in the incidence of rickets is due to the changing amount of solar ultraviolet radiation shorter that 313 millimicrons is now universally accepted. Despite the decrease in the antirachitic solar radiation during the winter months, it has been shown that sunshine possesses a definite antirachitic effect throughout the year. Tisdall and Brown1 in Toronto, Fleming2 in Washington, D. C., and Wilder and Vack3 in Boston found winter sunshine to be definitely antirachitic for rats even when passed through several of the substitutes for ordinary window glass. Caldwell and Dennett4 in New York and Wyman and his collaborators5 in Boston obtained similar results with infants. Similar studies have also demonstrated the seasonal variation in the ability of sunshine to prevent weakness of the legs in chickens.
The quality and intensity of the short ultraviolet radiations in sunlight are the determining factors in the
LAURENS H, MAYERSON HS. EFFICIENCY OF NEW ORLEANS SUNSHINE IN PREVENTING RICKETS IN RATS. Am J Dis Child. 1933;45(1):66-80. doi:10.1001/archpedi.1933.01950140076008