=Colour-extinction Pyrometers.=—Various attempts have been made to produce superposed glasses, or cells of coloured fluids, which will have the effect of extinguishing the colour of a heated source. As an example, three cells containing various dyes in solution may be prepared which, when looked through, will extinguish the colour at 840°, 850°, and 860° C. respectively. If it be desired to work at 850°, a difference of 10° on either side may be detected by a trained eye; but to follow a changing temperature a large number of cells would evidently be necessary. Heathcote’s extinction pyrometer, in its early form, consisted of an eye-shade in front of which two pairs of cells containing coloured fluid were mounted. In bringing a furnace to an assigned temperature, observation was made from time to time until a faint red image was perceived through one pair of cells, when the heat supply was regulated so as to maintain the existing temperature. When viewed through the second pair of cells, which contained a slightly darker fluid, no red image was to be seen at the correct temperature. With training, a workman could control a furnace to a fair degree of accuracy by this means, but the operation was tedious, and useful only for the attainment of a single temperature. In a later instrument, known as the “Pyromike” (fig. 63), Heathcote employs a single cell with flexible walls, so that by turning the screw-end, the length of the column of fluid interposed between the eye and the furnace can be altered. In taking a reading, the furnace is sighted and the screw turned so as to increase the length of the column of coloured fluid, until the image is no longer visible. A direct reading of the temperature is then obtained on a spiral scale marked on the cylindrical body of the instrument, over which the screwed portion rotates. This forms a simple and convenient temperature gauge for workshop use. [Illustration: FIG. 63.—HEATHCOTE’S EXTINCTION PYROMETER OR “PYROMIKE.”] [Illustration: FIG. 64.—“WEDGE” PYROMETER.]