http://pec.lib.wvu.edu/record/P11191939A Simple device for changing the light intensity in the x-ray illuminator. Journal article reprint from: <em>The Journal of Laboratory and Clinical Medicine</em>, v.24:no.8 (1939:May).Northup, David W.Van Liere, Edward J., 1895-1979.X-rays--Equipment and supplies.14256