The New Astrononty.—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 increase 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 growth 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 nebula: 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 positions 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, George B. and Alvan G. Their latest masterpieces are the huge 36-inch objec tive of the Lick telescope, and that of the Yerkes instrument.
In the matter of the polishing of opti cal surfaces, the figuring of lenses, and the ruling of gratings, American arti sans have excelled all others from their first attempts. Only in the production of instruments of precision and in the making of optical glass do they still yield superiority to European artisans.
Celestial Photography.—As early as 1840 Dr. John W. Draper, of New York, •itained a few photographs of the moon .bout an inch in diameter. In 1845, at Cambridge, Mass., Bond obtained photo graphic impressions of Vega and Castor, and in 1850 obtained the picture of the moon.
Among the successes of photography as an adjunct to the new astronomy have been photographs of stellar spectra by Pickering at the Harvard University Observatory, and the photographic nor mal spectrum of the sun recently com pleted by Rowland at the Johns Hopkins University.
The Mount Wilson (Cal.) Solar Ob servatory of the Carnegie Institute (1904), designed for spectroscopic study of the sun and stars, has produced marvelous photographs of these bodies that have proved invaluable to astron omers.