In recent papers it was shown by Hess and Unger that rickets in infants could be cured by frequent short exposures to the sun's rays.1, 2 By this means and without any alteration whatsoever of the dietary, the characteristic signs of this disorder begin to disappear in three to four weeks as noted by clinical examination and by the x-ray. As a result of favorable experiences of this nature, it was concluded in a study of the seasonal incidence of rickets3 that “hygienic factors, especially sunlight, and not dietetic factors play the dominant role in the marked seasonal variations of this disorder.” It seems probable that the ultra-violet rays play a large part in this curative power of the sun, judging from the work of Huldschinsky4 and others who recently have shown that infantile rickets can be cured by means of the rays produced by the mercury-vapor lamp. In 1918 we tried the curative effect of rays from this source, but, lacking the aid of x-ray examinations, could not convince ourselves of their effi...