As the intensity of the light emitted at the higher temperatures becomes dazzling, it is found desirable to introduce a piece of red glass in the eye piece at _R_. This also eliminates any question of matching colors, or of the observer's ability to distinguish colors. It is further of value in dealing with bodies which do not radiate light of the same composition as that emitted by a black body, since nevertheless the intensity of radiation of any one color from such bodies increases progressively in a definite manner as the temperature rises. The intensity of this one color can therefore be used as a measure of temperature for the body in question. Figures 124 to 126 show the way it is read. CORRECTION FOR COLD-JUNCTION ERRORS The voltage generated by a thermo-couple of an electric pyrometer is dependent on the difference in temperature between its hot junction, inside the furnace, and the cold junction, or opposite end of the thermo-couple to which the copper wires are connected. If the temperature or this cold junction rises and falls, the indications of the instrument will vary, although the hot junction in the furnace may be at a constant temperature. A cold-junction temperature of 75 deg.F., or 25 deg.C., is usually adopted in commercial pyrometers, and the pointer on the pyrometer should stand at this point on the scale when the hot junction is not heated. If the cold-junction temperature rises about 75 deg.F., where base metal thermo-couples are used, the pyrometer will read approximately 1 deg. low for every 1 deg. rise in temperature above 75 deg.F. For example, if the instrument is adjusted for a cold-junction temperature of 75 deg., and the actual cold-junction temperature is 90 deg.F., the pyrometer will read 15 deg. low. If, however, the cold-junction temperature falls below 75 deg.F., the pyrometer will read high instead of low, approximately 1 deg. for every 1 deg. drop in temperature below 75 deg.F. With platinum thermo-couples, the error is approximately 1/2 deg. for 1 deg. change in temperature. CORRECTION BY ZERO ADJUSTMENT.--Many pyrometers are supplied with a zero adjuster, by means of which the pointer can be set to any actual cold-junction temperature. If the cold junction of the thermo-couple is in a temperature of 100 deg.F., the pointer can be set to this point on the scale, and the readings of the instrument will be correct.