by the earth's semi-diameter. Efforts were made with indifferent success to fix its value by the aid of the transits of Venus in the eighteenth and nine- teenth centuries. The asteroids have proved more efficient auxiharics; and through the mediation of Iris. Sappho, and Victoria, in 1888-89, Sir David Gill assigned to the great unit of space a length of 92,800.000 miles, which the photographic measures of Eros, in 1900-01, bid fair to ratify. The stars, however, are so vastly remote that the only chance of detecting their perspective displacements is by observing them at intervals of six months, from opposite extremities of a base-line nearly 186.000 miles in extent. Thus, the annual parallax of a star means the angle under which the semi-diameter of the earth's orbit would be seen if viewed from its situation. This angle is in all eases, extremely minute, and in most cases, altogether evanescent; so that, from only about eighty stars (as at present known), the terrestrial orbit would appear to have sensible dimensions. Our nearest stellar neighbour is the splendid southern binary, a Centauri; yet its distance is such that hght needs four and one-third years to perform the jomney thence. Thomas Henderson (1798-18-14) announced his detection of its parallax in 18;39, just after Bessel of Konigsberg (1784-1846) had obtained a similar, but smaller result for an insignificant double star designated 61 Cygni.
Celesti.vl Photogr.vpht. — The second half of the nineteenth century was signalized by a revolu- tionary change in the methods and purposes of astronomy. Experiments in lunar photography, begun in 1840 by J. W. Draper of New York, were continued in the fifties l5y W. C, Bond, Warren de la Rue, and Lewis M. Rutherfurd. The first daguerre- otype of the sun was secured at Paris in 1845, and traces of the solar corona appeared on a sensitized plate exposed at Konigsberg during the total eclipse of 28 July, 1851. But the epoch of effective solar pliotography opened with the Spanish eclipse of 18 July, 1860, when the pictures successively ob- tained by Father Angelo Secchi, S.J., and Warren de la Rue demonstrated the solar status of the crim- son protuberances by rendering manifest the advance of the moon in front of them. At subsequent eclipses, the leading task of the camera has been the portrayal of the corona; and its importance was enhanced when \. C, Ranyard pointed out, in 1879, the corre- spondence of changes in its form with the alternations of solar disturbance. The eleven-year periodicity of sunspots was published in 1S51 by Schwabe of Dessau; and among the numerous associated phe- nomena of change, none are better ascertained than those affecting the shape of the silvcrj- aureola seen to encompass the sun when the moon cuts off the glare of direct sunlight. At spot maxima the aureola spreads its beamy radiance round the disc. But at times of minimum, it consists mainly of two great wings, extended in the sun's equatorial plane. A multitude of pliotographs. taken during the eclipses of 1898, 1900, 1901, and 1905, attest with certainty the punctual recurrence of these unexplained vicissi- tudes. The fundamental condition for the progress of sidereal photography is the use of long exposures; since most of the objects to be delineated emit light so feebly that its chemical effects must accumulate before they become sensible. But long exposures were impracticable until Sir William Huggins, in 1876, adopted the dn,--plate process; and this date, accordingly, marks the beginning of the wide- spreading ser\'iceableness of the camera to astronomy. In nebular investigations above all, it far outranges the telescope. Halley described in 1716 six nebu- la, which he held to be composed of a lucid medium collected from space. The Abbe Lacaille (1713-62) brought back with him from the Cape, in 1754. a list
of forty-two such objects; and Charles Messier (1730-1817) enumerated, in 1781, 103 nebula; an i clusters. But this harvest was scanty indeed con pared with the lavish yield of Herschel's exploration- Between 1786 and 1802 he communicated to the Royal Society catalogues of 2,500 nebula; he dis- tinguished their special forms, classified them in order of brightness, and elaborated a theory of stellar development from nebula, illustrated by selected instances of progressive condensation. The next considerable step towards a closer acquaintance with nebulae was made by Lord Rosse in 1845, wlien the prodigious light-grasp of his six-foot reflector afforded him the discoverj' of the great " V.'hirlpool'" structure in Canes Venatici. It proved to be tj-jiical of the entire class of spiral nebula, the large prev- alence of which has been one of the revelations of photography. The superiority in nebula-portraiture of the chemical to the eye-and-hand method was strikingly manifested in a photograph of the Orion nebula taken by Dr. A. A. Common, 30 Januarj-, 1883. Its efficacy for discovery became evident through the disclosure, on plates exposed by Paul and Prosper Henr\-, and by Isaac Roberts in 1885-86. of complex nebulous formations in the Pleiades, almost wholly invisible optically. Professor Keeler (1857-1900) estimated at 120.000 the number of nebula which the Crosslej' reflector of the Lick observatorj' would be capable of recording in both hemisplieres mth an hour's exposure, while tele- scopically constructed catalogues include less than 10,000. But it is through the combination of pho- tograpliy with spectroscopy, constituting the spectro- graphic mode of research, that astrophysics has achieved its most signal triumphs.
Astrophysics. — The fundamental principle of spectrum analysis, enunciated by Gustav Kirchhoff (1824-87) in 1859, depends upon the equivalence of emission and absorption. This means that, if white light be transmitted through glowing vapours, they arrest just those minute sections of it with which they themselves shine. And if the source of the white light be hotter than the arresting vapour, there results a prismatic spectrum, interrupted by dark lines, distinctive of the chemical nature of the substance originating them. Now this is exactly the case of the sun and stars. The white radiance emanating from their photospheres is found, when dispersed into a spectrum, to be crossed by numerous dusky rays indicating absorption by gaseous strata. to the c"ompo.sition of which Kirchhoff's principle supplies the clue. Kirchhoff himself identified in 1861, as prominent solar constituents, sodium, iron magnesium, calcium, and chromium; hydrogen was recognized by A. J. Angstrom (1814-74); helium by Sir Nonnan Lockj-er in 1868; and about forty elementarj- substances are now known with approxi- mate certainty to be common to the earth and sun. The chemistrj- of the stars is strictlj' analogous to that of the sun, although their spectra exliibit diver- sities s^nnptomatic of a considerable variety in physical state. Father Angelo Secchi. S.J. (1818^78), ba.sed on these diversities in 1863-67 a classification of the stars into four orders, still regarded as funda- mental, and supplied by Dr. Vogel in 1874 with an evolutionarj' interpretation, according to which, differences of spectral tj-pe are associated with various stages of progress from a tenuous and in- choate towards a compact condition. Since 1879, when Sir William Huggins secured impressions of an extended range of ultra-\'iolet white star light, stellar spectra have been mostly studied photo- graphically, the results being, not only precise and permanent, but also more complete than those obtain- able by visual means. The same eminent in\esti- gator discovered, in 1864, the bright-line spectra of certain clas.ses of nebula, by which they were known