=Mesuré and Nouel’s Pyrometer.=—This instrument, shown in fig. 62, consists of two Nicol prisms, between which is placed a piece of quartz cut perpendicularly to its axis. Light from the source, in passing through the first Nicol prism, is all polarised in the same plane; but on passing through the quartz is polarised in various planes, according to the wave-length. The colour seen after passing through the second prism, used as analyser, will depend upon the angle between this and the first or polarising prism. The analyser is connected to a rotating disc, divided into angular degrees; and on viewing the heated source the colour will appear red if the analyser be turned in one direction, and green if rotated in the opposite. The intermediate colour is a lemon-yellow; and the adjustment consists in rotating the analyser until this tint is obtained. The angular reading is then taken, and the temperature read off from a table prepared by making observations at known temperatures. Observers may disagree by as much as 100° C. in using this pyrometer, owing to differences in eyesight and judgment of the lemon-yellow tint; but a given operator, who has trained himself to the use of the instrument, may obtain much closer results with practice. The chief use of this device is to enable a judgment to be formed as to whether a furnace is above or below an assigned temperature, within limits of 25° C. on either side at the best; and hence it is convenient for a foreman or metallurgist to carry about for this purpose when other pyrometers are not in use. A great advantage is that the instrument is always ready for use, and has no accessories. [Illustration: FIG. 62.—MESURÉ AND NOUEL’S PYROMETER.]