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1Part of the expense incurred in assembling this apparatus was defrayed by a grant from the Council on Physical Therapy of the American Medical Association.

2Member of the Organisation Météorologique Internationale: Sub-Commission on the production of photoelectric cells for measuring biologically-effective ultraviolet radiation; Physicist, National Bureau of Standards, Washington, D. C.

3Professor of Physics, Northwestern University, Evanston, Ill.

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