An instrument in which heat radiated from the hot body is focused, by means of a concave mirror, on a small central hole behind which a small thermocouple is placed in front of two small, inclined mirrors. The instrument is sighted onto the hot body and focused by rotating a screw until the lower and upper halves of the image coincide; the electromotive force generated by the thermocouple is indicated on a galvanometer. The instrument, once focused, gives continuous readings and may be connected to a recording indicator.