Tests were made of the effect of several variables on the performance of the Eppley pyrheliometer. The tests showed: (1) How the output increased with decreasing ambient temperature; (2) how output varied with angle of incidence of collimated radiation; (3) that output decreased about 5 percent when receiver was exposed in the vertical plane, but that complete inversion from the horizontal had no significant effect; and (4) that a few water droplets on the glass envelope did not influence output. In addition, spectral transmission data, from National Bureau of Standards tests, are shown.