The “Wedge” Pyrometer, designed by Alder and Cochrane (fig. 64), consists of a small telescope through which a prism of darkened glass may be moved, and which is focused on the heated object. By turning a head the wedge may be moved so as to interpose a thicker layer of dark glass between the eye and the furnace, and the same operation causes a temperature scale to pass in front of a fixed pointer. When the image of the hot source is just extinguished, the temperature is read from the mark opposite the fixed point. Training is needed to enable an observer to judge the exact point of extinction, when it becomes possible to obtain results of 20° C. in the region of 1300° C. On the other hand, when used by one unaccustomed to the instrument, the reading may be wrong by 50° C. or more. As an aid to the judgment near the extinction point, the hand may be interposed between the telescope and furnace, when, if extinction be complete, no alteration should be observed in the field of view. The simple construction of this pyrometer is an advantage, no accessories being needed; and when used with the precautions stated above, readings sufficiently close for many processes can easily be obtained. =Management of Optical Pyrometers.=—Careful usage is essential with optical pyrometers, which are liable to get out of adjustment with rough handling; and for this reason a trained observer should be in charge of such instruments. Skilled attention is equally requisite in taking readings, as the matching of tints correctly is an operation which demands a high degree of judgment. Careful attention must be paid to the standard lights; if flames, regulation to the standard height is essential; if electric lamps, care must be taken not to use them for a longer period than necessary, in order to increase the useful life. Accumulators should be recharged regularly—say once in two weeks—to keep in good order. Separate parts, such as absorption glasses, should be kept in a place of safety, as their destruction may involve a new calibration. It should be kept in mind that the temperatures indicated by optical pyrometers are “black” temperatures; that is, they correspond to the readings that would be given by a black-body of the same degree of brightness. In consequence, readings should always be taken under black-body conditions, the precautions in this respect being identical with those necessary for total-radiation pyrometers, given on page 163. In some special cases the connection between the apparent and true temperatures has been worked out for a given type of pyrometer, but, owing to the different emissive powers of different substances, no general relation can be given.