a further recommendation. The radiation method, however, is not suited to the purposes of an installation, as even if mirrors and junctions could be constructed so as to be identical, the arrangement would be very costly. A cheap adaptation of the radiation principle, by means of which a number of furnaces, such as a set of cement-kilns, could be controlled from a centre, would be of great advantage, and would add further to the general utility of this class of pyrometer. CHAPTER VI OPTICAL PYROMETERS =General Principles.=—When a solid is heated to 450° C., it commences to send out luminous radiations and appears a dull-red colour in a darkened room. As the temperature rises, the luminous radiations become more intense; the colour changes to a lighter red, then to orange, yellow, white, and finally to a dazzling white. Attempts have been made to assign temperatures to specified colours, and Pouillet, in 1836, introduced a table which purported to give the relation between colour and temperature. The following table, published by Howe in 1900, differs considerably from that of Pouillet, who had no accurate means of measuring the temperatures he assigned to the colours:— HOWE’S TABLE. ───────────────────────────────┬───────────────┬────────────── Description. │ Temp. Deg. C. │ Temp. Deg. F. ───────────────────────────────┼───────────────┼────────────── Lowest red visible in darkness │ 470 │ 878 ” ” ” daylight │ 475 │ 887 Dull red │ 550 to 625 │ 1022 to 1157 Full cherry │ 700 │ 1292 Light red │ 850 │ 1562 Full yellow │ 950 to 1000 │ 1742 to 1832 Light yellow │ 1050 │ 1922 White │ 1150 │ 2108 ───────────────────────────────┴───────────────┴────────────── If it were possible for all observers to detect exactly the colours to which these temperatures refer, the table would be of great utility; but in practice any two persons might differ in judgment to the extent of 50° C. below a yellow; and when the white is reached, and becomes dazzling, accurate discrimination is impossible. At the same time, a trained workman, used to quenching steel at a fixed temperature, say 850° C., acquires a high degree of judgment with constant practice, and may not vary by more than 20° C. at temperatures below a light yellow. The personal equation, however, is too great for colour judgment by the unaided eye to be taken as an accurate guide to temperature. A fairly close approximation, however, may be obtained by matching the colours against prepared standards, as will be referred to later.