Page:Collected Physical Papers.djvu/380

360 light reflected from the attached mirror. The sensitiveness of the apparatus is very greatly enhanced by the employment of a perfect system of astatic needles.

This method for the measurement of radiation is extraordinarily sensitive. For instance, there is an apparatus permanently adjusted in the Institute in which the sensitive element is a thin strip of ebonite which is enclosed in a tube with a narrow slit in front. When any person walks at some distance past the instrument, the indicating spot of light remains perfectly quiescent until the individual is in line of sight of the sensitive strip; a sudden deflection is then produced by the radiation which is emitted from the human body, through thick warm clothing and a heavy over-