=Wanner’s Pyrometer.=—The principle of this pyrometer is the comparison of the brightness of a red ray from the standard with that of the ray of some wave-length obtained from the source, both rays being produced spectroscopically and therefore being truly monochromatic. The brightness is compared by the aid of a polarising device, resulting in a somewhat complicated optical arrangement, which is shown in fig. 57. Light from a standard electric lamp passes through the slit S_{1}, and from the hot source through S_{2}. Both beams are rendered parallel by means of an achromatic lens O_{1}, which is placed at a distance equal to its focal length from the slits. The parallel beams are dispersed by the direct-vision spectroscope P; and then pass through the polarising prism R, which separates each beam into two beams, polarised in planes at right angles. A biprism, B, placed in contact with a second achromatic lens, O_{2}, is made of such an angle that two fields of red light, polarised in planes at right angles, one from the source and the other from the standard, are focused on the slit D. These fields are viewed through an analyser A, and are brought to equal brightness by rotating the analyser, to which a graduated scale is attached, the temperature being deduced from the angle through which the analyser is turned. The calibration is effected by Wien’s law (equation (3) page 172), the intensities of standard and source being related to the angle of rotation as indicated by the equation. J_{2}/J_{1} = tan^2 Θ where J_{2} and J_{1} represent the intensities of source and standard respectively, and Θ = angle of rotation. Introducing this value into Wien’s equation (page 172), the relation between Θ and T may be shown to take the form log tan Θ = _a_ + _b_/T, where _a_ and _b_ are constants. Hence, if log tan Θ be plotted against 1/T a straight line is obtained, and hence by a few observations at known temperatures a calibration curve may be drawn from which intermediate and extraneous readings may be obtained. Messrs Hadfield have introduced a special chart, divided so that actual readings in degrees C. may be taken directly by observing the angle Θ. As sent out for use, the temperature scale is prepared beforehand, so that direct readings may be taken. As the standard electric lamp will vary in brightness with repeated use, means must be provided to restore it to its proper value. This can be done by placing a rheostat in the circuit of the lamp, and adjusting the current until the brightness, as viewed through the pyrometer, exactly agrees with that of a ground-glass surface illuminated by a standard amyl-acetate lamp. The flame of this lamp really constitutes the standard; but as it would be blown about by air-currents when used in a workshop, the electric lamp, lighted by a portable battery, is brought to equality and used for general measurements.