A successful beginning was made in nebular spectrography by Sir William Huggins, March 7, 1882.[1537] Five lines in all stamped themselves upon the plate during forty-five minutes of exposure to the rays of the strange object in Orion. Of these, four were the known visible lines, and a fifth, high up in the ultra-violet, at wave-length 3,727, has evidently peculiar relationships, as yet imperfectly apprehended. It is strong in the spectra of many planetaries; it helped to characterise the nebular metamorphosis of Nova Aurigæ, yet failed to appear in Nova Persei. Two additional hydrogen lines, making six in all, were photographed at Tulse Hill, from the Orion nebula, in 1890;[1538] and Dr. Copeland's detection in 1886[1539] of the yellow ray D3 gave the first hint of the presence of helium in this prodigious formation. Nor are there wanting spectroscopic indications of its physical connection with the stars visually involved in it. Sir William and Lady Huggins found a plate exposed February 5, 1888, impressed with four groups of fine bright lines, originating in the continuous light of two of the trapezium-stars, but extending some way into the surrounding nebula.[1540] And Dr. Scheiner[1541] argued a wider relationship from the common possession, by the nebula and the chief stars in the constellation Orion, of a blue line, bright in the one case, dark in the others, since identified as a member of one of the helium series.
The structural unity of the stellar and nebular orders in this extensive region of the sky has also, by direct photographic means, been unmistakably affirmed.
The first promising autographic picture of the Orion nebula was obtained by Draper, September 30, 1880.[1542] The marked approach towards a still more perfectly satisfactory result shown by his plates of March, 1881 and 1882, was unhappily cut short by his death. Meanwhile, M. Janssen was at work in the same field from 1881, with his accustomed success.[1543] But Dr. A. Ainslie Common left all competitors far behind with a splendid picture, taken January 30, 1883, by means of an exposure of thirty-seven minutes in the focus of his 3-foot silver-on-glass mirror.[1544] Photography may thereby be said to have definitely assumed the office of historiographer to the nebulæ, since this one impression embodies a mass of facts hardly to be compassed by months of labour with the pencil, and affords a record of shape and relative brightness in the various parts of the stupendous object it delineates which must prove invaluable to the students of its future condition. Its beauty and merit were officially recognised by the award of the Astronomical Society's Gold Medal in 1884.
A second picture of equal merit, obtained by the same means, February 28, 1883, with an exposure of one hour, is reproduced in the frontispiece. The vignette includes two specimens of planetary photography. The Jupiter, with the great red spot conspicuous in the southern hemisphere, is by Dr. Common. It dates from September 3, 1879, and was accordingly one of the earliest results with his 36-inch, the direct image in which imprinted itself in a fraction of a second, and was subsequently enlarged on paper about twelve times. The exquisite little picture of Saturn was taken at Paris by MM. Paul and Prosper Henry, December 21, 1885, with their 13-inch photographic refractor. The telescopic image was in this case magnified eleven times previous to being photographed, an exposure of about five seconds being allowed; and the total enlargement, as it now appears, is nineteen times. A trace of the dusky ring perceptible on the original negative is lost in the print.
A photograph of the Orion nebula taken by Dr. Roberts in 67 minutes, November 30, 1886, made a striking disclosure of the extent of that prodigious object. More than six times the nebulous area depicted on Dr. Common's plates is covered by it, and it plainly shows an adjacent nebula, separately catalogued by Messier, to belong to the same vast formation.
This disposition to annex and appropriate has come out more strongly with every increase of photographic power. Plates exposed at Harvard College in March, 1888, with an 8-inch portrait-lens (the same used in the preparation of the Draper Catalogue) showed the old-established "Fish-mouth" nebula not only to involve the stars of the sword-handle, but to be in tolerably evident connection with the most easterly of the three belt-stars, from which a remarkable nebulous appendage was found to proceed.[1545] A still more curious discovery was made by W. H. Pickering in 1889.[1546] Photographs taken in three hours from the summit of Wilson's Peak in California revealed the existence of an enormous, though faint spiral structure, enclosing in its span of nearly seventeen degrees the entire stellar and nebulous group of the Belt and Sword, from which it most likely, although not quite traceably, issues as if from a nucleus. A startling glimpse is thus afforded of the cosmical importance of that strange "hiatus" in the heavens which excited the wonder of Huygens in 1656. The inconceivable attenuation of the gaseous stuff composing it was virtually demonstrated by Mr. Ranyard.[1547]
In March, 1885, Sir Howard Grubb mounted for Dr. Isaac Roberts, at Maghull, near Liverpool (his observatory has since been transferred to Crowborough in Sussex), a silver-on-glass reflector of twenty inches aperture, constructed expressly for use in celestial photography. A series of nebula-pictures, obtained with this fine instrument, have proved highly instructive both as to the structure and extent of these wonderful objects; above all, one of the great Andromeda nebula, to which an exposure of three hours was given on October 1, 1888.[1548] In it a convoluted structure replaced and rendered intelligible the anomalously rifted mass seen by Bond in 1847.[1549] The effects of annular condensation appeared to have stamped themselves upon the plate, and two attendant nebulæ presented the aspect of satellites already separated from the parent body, and presumably revolving round it. The ring-nebula in Lyra was photographed at Paris in 1886, and shortly afterwards by Von Gothard with a 10-inch reflector,[1550] and he similarly depicted in 1888 the two chief spiral and other nebulæ.[1551] Photographs of the Lyra nebula taken at Algiers in 1890,[1552] and at the Vatican observatory in 1892,[1553] were remarkable for the strong development of a central star, difficult of telescopic discernment, but evidently of primary importance to the annular structure around.
The uses of photography in celestial investigations become every year more manifold and more apparent. The earliest chemical star-pictures were those of Castor and Vega, obtained with the Cambridge refractor in 1850 by Whipple of Boston under the direction of W. C. Bond. Double-star photography was inaugurated under the auspices of G. P. Bond, April 27, 1857, with an impression, obtained in eight seconds, of Mizar, the middle star in the handle of the Plough. A series of measures from sixty-two similar images gave the distance and position-angle of its companion with about the same accuracy attainable by ordinary micrometrical operations; and the method and upshot of these novel experiments were described in three papers remarkably forecasting the purposes to be served by stellar photography.[1554] The matter next fell into the able hands of Rutherfurd, who completed in 1864 a fine object glass (of 11-1/2 inches) corrected for the ultra-violet rays, consequently useless for visual purposes. The sacrifice was recompensed by conspicuous success. A set of measurements from his photographs of nearly fifty stars in the Pleiades, and their comparison with Bessel's places, enabled Dr. Gould to announce, in 1866, that during the intervening third of a century no changes of importance had occurred in their relative positions.[1555] And Mr. Harold Jacoby[1556] similarly ascertained the fixity of seventy-five of Rutherfurd's Atlantids, between the epoch 1873 and that of Dr. Elkin's heliometric triangulation of the cluster in 1886,[1557] extending the interval to twenty-seven years by subsequent comparisons with plates taken at Lick, September 27, 1900.[1558] Positive, however, as well as negative results have ensued from the application of modern methods to that antique group.
On October 19, 1859, Wilhelm Tempel, a Saxon peasant by origin, later a skilled engraver, discovered with a small telescope, bought out of his scanty savings, an elliptical nebulosity, stretching far to the southward from the star Merope. It attracted the attention of many observers, but was so often missed, owing to its extreme susceptibility to adverse atmospheric influences, as to rouse unfounded suspicions of its variability. The detection of this evasive object gave a hint, barely intelligible at the time, of further revelations of the same kind by more cogent means.