There is a period of about eight months each year during which the amount of ultraviolet rays reaching the ground strata of atmosphere in our large cities is much diminished.
The problem of supplying children with ultraviolet radiation on a mass scale stimulated the building of the apparatus about to be described.
The first installation designed was for the Spalding School for Crippled Children, in which a large number of children had bone tuberculosis; it was purchased by the board of education.
A carbon arc unit was first considered. The idea, however, was discarded because of the fact that according to city ordinance, each carbon arc unit must have an operator in charge. In addition, the amount of heat generated by a carbon arc unit is very large and the treatment period, as time went on, would necessitate long exposure. The length of the exposure would, of course, curtail mass