[Illustration: FIG. 48.—FÉRY’S SPIRAL PYROMETER. FRONT VIEW.] The creeping movement is probably due to the whole instrument, and the air in the interior, becoming heated by the entering rays, and by proximity to the hot source. In a number of trials made by the author, it was noticed that when the instrument was allowed to stand near the furnace for some time before using, thereby attaining the temperature existing in the vicinity, the “creep” almost entirely vanished. All things considered, the spiral form of Féry’s pyrometer must be regarded as more portable but less accurate than that in which the rays are received on a thermal junction. =Foster’s Fixed-Focus Radiation Pyrometer.=—The necessity for focusing, common to all Féry’s radiation pyrometers, is obviated in Foster’s pyrometer, which, however, cannot be used from so great a distance. The principle involved in the fixed-focus pyrometer is that the amount of energy received by a concave mirror and focused on a thermal junction will not vary so long as the area of the surface sending rays to the mirror, through a fixed opening, increases as the square of the distance. This will be understood from fig. 49, in which C is the mirror, D a thermal junction fixed so as to be in the focus of the opening E F, and A B the heated surface. The lines joining E and F to the edge of the mirror intersect in a point G, and provided the lines G E and G F, if produced, fall within the heated surface A B, the quantity of energy falling on D will always be the same. A cross section of the cone G A B is a circle; and if A B be twice as far away from G as E F, the areas of the circles of which A B and E F are diameters will be in the ratio 4: 1. But as A B is twice as far from G as E F, the intensity of its radiations will be as 1: 4; and hence loss of radiating power is exactly balanced by increase in area. [Illustration: FIG. 49.—PRINCIPLE OF FOSTER’S FIXED-FOCUS PYROMETER.] In the actual instrument the tube in which the mirror is placed is blackened internally, so that no rays reach the mirror by reflection from it. The diameters of the opening E F and the mirror C are such that the perpendicular from G on to A B is ten times the length of A B. Hence, if the heated object be 6 inches in diameter, the limiting distance of G is 10 × 6 = 60 inches. The position of the point G is indicated by a ring on the outside of the tube, and in taking a measurement the tube is brought well within the distance prescribed, which is in all cases ten times the diameter of the heated object. Temperatures are read from a galvanometer connected to the thermal junction, the whole arrangement being portable, as shown in figs. 50 and 51, which represent the instrument in use. [Illustration: FIG. 50.—FOSTER’S PYROMETER, MOUNTED ON STAND.]