[ [12] Sir W. Herschel’s great undertakings, Bessel remarks (“Populäre Vorlesungen,” p. 15), “were directed rather towards a physical description of the heavens, than to astronomy proper.”
[ [13] Am. Jour. of Science, vol. xiii. p. 89.
[ [14] The characteristic orange line (D3) of this unknown substance, has recently been identified by Professor Palmieri in the spectrum of lava from Vesuvius—a highly interesting discovery, if verified.
[ [15] The Sun, p. 193.
[ [16] R. D. Cutts, “Bulletin of the Philosophical Society of Washington,” vol. i. p. 70.
[ [17] This instrument may be described as an electric balance of the utmost conceivable delicacy. The principle of its construction is that the conducting power of metals is diminished by raising their temperature. Thus, if heat be applied to one only of the wires forming a circuit in which a galvanometer is included, the movement of the needle instantly betrays the disturbance of the electrical equilibrium. The conducting wires or “balance arms” of the bolometer are platinum strips 1/120th of an inch wide and 1/25000 of an inch thick, constituting metallic antennæ sensitive to the chill even of the fine dark lines in the solar spectrum, or to changes of temperature estimated at 1/100000 of a degree Centigrade.
[ [18] Defined by the tint of the second hydrogen-line, the bright reversal of Fraunhofer’s F. The sun would also seem—adopting a medium estimate—three or four times as brilliant as he now does.
[ [19] Annales de Chimie et de Physique, t. x. p. 360.
[ [20] S. P. Langley, “Nature,” vol. xxvi. p. 316.
[ [21] Sir J. Herschel’s estimate of the “temperature of space” was 239°F.; Pouillet’s 224°F. below zero. Both are almost certainly much too high. See Taylor, “Bull. Phil. Soc. Washington,” vol. ii. p. 73; and Croll, “Nature,” vol. xxi, p. 521.