B y A R T H U R W . E R S K I N E , M . D . , Cedar Rapids , Iowa. T H E O R E T I C A L L Y , it is possible to determine what filters approach A most nearly the ideal of absorbing the maximum of long waves and the minimum of short ones, by interposing various materials in the path of a beam of X rays, produced under fixed conditions, and then by analysis of the spectra of the filtered beams, s tudy the effects of the filters upon wave-length. Such a method presents numerous technical difficulties, and a simpler and more direct solution of the problem is available. In practice, we endeavour to produce a beam of X rays with a high proportion of short wave-lengths, not because there is any especial