=Féry’s Optical Pyrometer.=—This instrument (shown in figs. 55 and 56) consists of a telescope furnished with a side-branch, in which a standard lamp E is placed. Light from E is focused upon a piece of transparent glass F, inclined at an angle of 45° to the axis of the telescope, from whence it is reflected into the eye-piece. To render the light received from the lamp monochromatic, a piece of red glass is interposed between E and the mirror. The telescope is sighted on the hot substance, rays from which pass through a piece of red glass D, and thence through two wedges of darkened glass, which diminish the intensity to a greater or less degree according to the thickness of absorbent glass interposed, which is reduced by sliding the wedges apart, and increased by the contrary movement. After passing through the wedges, the light proceeds through the inclined mirror to the eye-piece; consequently, the appearance presented to the eye is that of a field illuminated one-half by the standard lamp, and the other by the hot source. The adjustment consists in sliding the wedges, by a screw movement, until both portions of the field are equally illuminated. A temperature scale is provided on the moving piece which actuates the wedges, and is derived by Wien’s equation from the thickness of the wedges interposed when equality is obtained. Calibration is effected by noting the thickness of the wedges corresponding to two known temperatures, from which a straight line connecting thickness with the reciprocal of the absolute temperatures may be drawn, and a table formed giving values of T in terms of the thickness of the wedges. The calibration may be extended indefinitely, the accuracy of the readings depending upon the truth of Wien’s law. Féry’s optical pyrometer is a convenient instrument for occasional readings of high temperatures, combining simplicity with portability. =Le Chatelier’s Optical Pyrometer.=—This pyrometer was the original form of instrument in which the temperature of a luminous source was deduced by photometric comparison with a standard light; and Féry’s apparatus, described above, is merely a convenient modification of the original. Instead of the absorbent glass wedges, Le Chatelier employed an iris diaphragm to reduce the quantity of light entering the telescope; the adjustment being carried out by altering the size of the opening in the diaphragm until the brightness of the source agreed with that of the standard. The intensity of the light received in the telescope will vary as the square of the diameter of the opening; and calibration at two known temperatures with a given monochromatic glass enables a temperature scale corresponding to diameter of opening to be computed by Wien’s law. Le Chatelier’s pyrometer is a valuable implement for research work in the laboratory, but is not so convenient for workshop purposes as Féry’s modification. [Illustration: FIG. 57.—WANNER’S PYROMETER. SECTION.]