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LEFT ASTRONOMY 321 ASTRONOMY positions, magnitudes, to the nearest 10th, or 7,730 stars situated between — 10° and the South Pole, and the mag- nitudes, to the nearest quarter, of more than 1,000 others, mostly companions of these, or situated in clusters, the joint light of which equaled a seventh mag- nitude star. The New Astronomy. — The spectro- scope has been the principal instrument of investigation in the new astronomy. After the work of Kirchhoff and Bun- sen, the next important step was the investigation, with the diffraction spec- troscope, by Angstrom and Thalen, of the formation of the so-called normal spectrum, in which the distances of the lines are proportional to their wave- lengths. The map of the solar spec- trum constructed in this way has been the standard for the wave-lengths of the Fraunhofer line until within a very few years. The work of Rowland at the Johns Hopkins University, photograph- ing directly the spectrum formed from his concave mirror-gratings (partly in- vented by him), has so far exceeded the Angstrom maps that the latter may now be considered displaced. The phenom- ena attending the solar eclipses and of comets offered a new field for the spec- troscope, and in this a host of names at once claim attention, principal among which are those of Young, Hale, direc- tor of Yerkes, Keeler of Lick's, Vogel, Secchi, Huggins, Lockyer, Janssen, and Langley. The simultaneous and independent discoveries by Lockyer and Janssen in connection with the Indian solar eclipse of August, 1868 (that the solar promi- nences, or hydrogen clouds surrounding the sun, can be studied at any time without the help of an eclipse), revolu- tionized the methods of studying that part of the sun's surroundings. Photometry, or the measurement of the brightness of the different heavenly bodies, so far as its results are con- cerned, is properly classed under the new astronomy. It has, however, been employed from the earliest times, with- out instrumental assistance, in classify- ing the stars into a scale of magnitudes, and in later days in observation of the changes in the light of the variable stars. Solar Investigations. — Sir John Her- schel and Pouillet were the first to meas- ure the amount of heat which we receive from the sun by noting the inci'case in the temperature of a given amount of water upon which a given beam of sunlight is allowed to fall for a certain time. Using various forms of equivalent apparatus, Waterston, Erics- son, Secchi, Crova, Violle, Langley, and others have made different determina- tions of the so-called "solar-constant," or the amount of radiant energy which falls upon a square meter of surface at the upper limits of the atmosphere. The most remarkable work of all in the domain of radiant energy has been that of Langley with his bolometer. By means of this instrument minute amounts of such radiations, which were entirely beyond the reach of all previous experiments, can be detected and accu- rately measured. Further Progress. — In summarizing the grov/th of astronomy during the 19th century we enumerate the researches of Henderson, Winnecke, Brunnow, Gill, and Elkin in stellar parallax; the double-star discoveries and measures of Struve (Otto), Dawes, Dembowski, Burnham, and Stone; the discoveries of comets by Pons, Tuttle, Tempel, Swift, Brooks, Barnard, and many others; the discovery and cataloguing of nebulae by Herschel, Lassel, Tempel, Swift, Stone, and Dreyer; the elaborate work of Car- rington on sun spots and the positions of northern circumpolar stars; the charting of faint ecliptic stars by Cha- cornac, the Henry brothers, and espe- cially Peters; Chandler's important work in variable stars and in variation of latitude; the work of Schmidt on va- rious stars and in selenography; the discovery of difficult planetary satellites by Lassel and Bond; the spectroscopic researches of Young, Schuster, Draper, Thollon, and Lohse; the determinations of the velocity of light by Fizeau, Fou- cault, Michelson, and Newcomb; Gill's work upon the parallax of Mars and some of the asteroids; Elkin's thorough remeasurement of the position of the stars of the Pleiades with the heliom- eter; Darwin's investigation of the entirely new subject of the bearing of tidal friction upon the development of planetary and satellite systems and Stone's observations at the Cape, re- sulting in the formation of the "Cape Catalogue," which ranks next to the work of Gould in furnishing us exact Ipositions of the stars of the southern heavens. Harkness' work upon the re- duction of the American observations of the transit of Venus should also be noted. Instruments. — The history of the prog- ress of astronomy in the 19th century would be incomplete without a mention of the remarkable opticians and mech- anicians whose handiwork has made it possible. We have already mentioned Fraunhofer. Pre-eminent among them all are the names of the late Alvan Clark, of Cambridge, Mass., and his sons.