The instrument is light and portable, and can be sighted as easily as an opera glass. The telescope, which is held in the hand, weighs only 25 oz.; and the case containing the battery, rheostat and milliammeter, which is slung from the shoulder, only 10 lb. [Illustration: FIG. 119.--Thermocouple in floor of furnace.] [Illustration: FIG. 120.--Pyrometer in gas furnace.] A large surface to sight at is not required. So long as the image formed by the objective is broader than the lamp filament, the temperature can be measured accurately. [Illustration: FIG. 121.--Tycos multiple indicating pyrometer and recorder.] [Illustration: FIG. 122.--Pyrometer in galvanizing tank.] Distance does not matter, as the brightness of the image formed by the lens is practically constant, regardless of the distance of the instrument from the hot object. [Illustration: FIG. 123.--Leeds & Northrup optical pyrometer.] The manipulation is simple and rapid, consisting merely in the turning of a knurled knob. The setting is made with great precision, due to the rapid change in light intensity with change in temperature and to the sensitiveness of the eye to differences of light intensity. In the region of temperatures used for hardening steel, for example, different observers using the instrument will agree within 3 deg.C. [Illustration: FIG. 124.--Too low. FIG. 125.--Too high. FIG. 126.--Correct.] Only brightness, not color, of light is matched, as light of only one color reaches the eye. Color blindness, therefore, is no hindrance to the use of this method. The use of the instrument is shown in Fig. 127. OPTICAL SYSTEM AND ELECTRICAL CIRCUIT OF THE LEEDS & NORTHRUP OPTICAL PYROMETER.--For extremely high temperature, the optical pyrometer is largely used. This is a comparative method. By means of the rheostat the current through the lamp is adjusted until the brightness of the filament is just equal to the brightness of the image produced by the lens _L_, Fig. 123, whereupon the filament blends with or becomes indistinguishable in the background formed by the image of the hot object. This adjustment can be made with great accuracy and certainty, as the effect of radiation upon the eye varies some twenty times faster than does the temperature at 1,600 deg.F., and some fourteen times faster at 3,400 deg.F. When a balance has been obtained, the observer notes the reading of the milliammeter. The temperature corresponding to the current is then read from a calibration curve supplied with the instrument. [Illustration: FIG. 127.--Using the optical pyrometer.]