'444 



NA rURE 



[March io, 1898 



the wonderful extensions round the Pleiades. His work in ! 

 celestial photography has been so fruitful in results, that a brief [ 

 record of its growth will not be out of place. In 1883, Dr. j 

 Roberts made a series of experiments to test the suitability of 

 ordinary photographic lenses for the delineation of celestial 

 objects ("Photographs of Stars, Star-Clusters, and Nebula," I. 

 Roberts, 1894). The results were so promising that he deter- 

 mined to develop the photographic method of observation, and, 

 with this end in view, he ordered to be constructed a reflecting 

 ■ telescope of twenty inches diameter, and one hundred inches 

 focal length. The instrument was ready for use in April 1885, 

 and work was then commenced with it. But it was not until 

 more than a year later that its performances began to reach Dr. 

 Roberts's expectations. Only those who have had to coax 

 astronomical instruments into a tractable condition, can under- 

 stand and appreciate the difficulties with which he had to con- 

 tend. It was found that satisfactory photographs could not be 

 obtained until the driving-clock of the telescope had been greatly 

 improved in regard to its ability to keep the instrument accurately 

 following the apparent movements of the stars. When a sensi- 

 ; tive plate has to be exposed to starlight for three or four hours, 

 the beams from individual stars must continually beat upon the 

 same spot, otherwise the stellar images do not appear as circular 

 •discs upon the resulting picture. To attain this desideratum — 

 that is to say, to make a clock capable of driving a telescope so 



Fig. 



-The Pleiades Nebuln. (From a photograph by Dr. Roberts, with 

 an exposure of ten hours.) 



as to keep absolutely the same time as the stars — taxes the 

 instrument-maker to his utmost ; and, when Dr. Roberts com- 

 menced his work, there was no controlling clock that completely 

 supplied the want. His early photographs, therefore, did not 

 exhibit the stars as perfectly circular spots, nevertheless they 

 greatly extended the state of knowledge of the Pleiades nebu- 

 losities. In October 1886, the first of his remarkable long- 

 exposure photographs was obtained, the sensitive plate being 

 kept directed to the Pleiades for three hours. With regard to 

 the amount of nebulosity shown, the resulting picture was far in 

 advance of all previous ones. It demonstrated that the MM. 

 Henry had merely picked up the fringe of the nebulous matter 

 round the Pleiades, for not only were all the «stars of the cluster 

 shown to be shrouded in nebulae, but, to quote Dr. Roberts's 

 words (Mjnthly Notices, R.A.S., vol. xlvii. p. 24, 1886-7), 

 ^'the nebulosity extends in streamers and fleecy masses, till it 

 seems almost to fill the spaces between the stars, and to extend 

 far beyond them. It suj;;gests the probability that these stars in 

 the Pleiades, toT;ether with many of the stars around them, are 

 involved either directly or else in slight alignment with one vast 

 nebula." This surmise was more than justified by future 

 results. A year after it was made, the Henrys succeeded in 



obtaining a photograph which confirmed it. They, like Dr. 

 Roberts, had been working towards perfecting their new engine 

 of research. By the use of more sensitive plates, and by 

 increasing the duration of exposure, they were able to bear wit- 

 ness that the feeble traces of nebulosity shown around tfae stars 

 Maia, Merope, and Electra, on the views taken two years 

 previously, appeared on the new photographs as bright nebulous 

 masses, all more or less merging into one another, while »an 

 extensive lucid patch surrounded Alcyone (Observatoire de 

 Paris, Rapport Annuel, 1887). Threads of nebulosity were 

 also revealed, projecting from the central mass and having 

 stars strung upon them like beads on a rosary. 



Until recently, the best result of Pleiades photography was 

 reached by Dr. Roberts at the end of 1888 {Monthly Notices, 

 R.A.S., vol. xlix. p. 120, 1888-89). The view of the wonderful 

 group then obtained was produced by exposing the sensitive 

 plate for four hours. It showed more clearly and more beautifully 

 the filmy sea in which the stars are immersed, but did not 

 extend the limits defined by the earlier pictures. The ne plus 

 ultra of Pleiades photography was lately obtained by Dr. 

 Roberts with an exposure of ten hours, and a reproduction of 

 the picture is here given (Fig. i). No process of reproduction 

 can, however, do justice to the brilliant picture which Dr. 

 Roberts has permitted us to reproduce. 



The epoch-making views of the Pleiades obtained ten years 

 ago gave a great impetus to nebular photography. The thought 

 that for a time to be reckoned in reons, the gauze-like veil On 

 which the Pleiades were studded had been unable to aff"ect man's 

 visual perceptions, but could register its existence automatically 

 upon a prepared surface, was sufficient to open up a vista of 

 marvellous possibilities. 



Dr. Roberts's photograph of the Pleiades nebula has become 

 a classic. Of his numerous other photographs of nebulae, three 

 have attained a like eminence ; they represent the Great Nebula 

 in Orion (to which reference has previously been made), the 

 Great Nebula in Andromeda, and the Spiral Nebula in the con- 

 stellation of Canes Venatici, and to these attention must here 

 be confined. It is perhaps invidious to select these pictures from 

 the collection of five hundred photographs of nebulae and clusters 

 with which Dr. Roberts has enriched astronomy, for every one 

 of his photographs adds to what was previously knovvn about the 

 objects portrayed. The nebulae above designated, however, 

 were all made to record their own forms at about the same 

 time, and each of the monochromes produced exhibited striking 

 novelties. 



The Orion Nebula. 



A photograph of the Orion nebula taken by Dr. Roberts in 

 November 1887, though only representing a tentative result, 

 showed nebulous matter covering an area quite seven times 

 greater than that covered by Dr. Common's photograph (M?«Mi^ 

 Notices, R. A. S., vol. xlvii. p. 89). 



What Dr. Roberts had gained in extent, however, he had 



lost in detail. The delicate structure of the central part of the 



nebula appeared but a splash of white upon the print presented 



to the Royal Astronomical Society, though it is fair to add that 



they could be distinguished upon the negative. The lesson 



learned was, that it was extremely difficult to obtain upon one 



and the same print a representation of the nebula showing it to 



the fullest extent, and also exhibiting the fine markings and 



imperceptible gradations that characterise such objects. An 



increase of the time of exposure certainly increases the area of 



j nebulosity depicted upon a photographic plate; but while the 



picture is having its limits extended, the filmy rays in the brighter 



regions of nebulosity are being "burnt out " by the cumulative 



action. This difficulty has now been overcome ; for since Dr. 



Roberts revealed to astronomers the vast extent of the Orion 



nelnila, he has taken, upon a single plate, photographs which 



show the fine detail of the nebulous structure as well as the great 



sky-area covered by the nebulous mass. One of these pictures, 



I taken with a dual exposure of seven hours thirty-five minutes, 



j on two evenings in February 1894, is here reproduced (Fig. 2), 



I and it represents, according to Dr. Roberts, " the maximum 



of extent and detail that can be shown by aid of photographic 



methods." The illustration is, however, but a feeble represent- 



i ation of the original picture. Longer exposure with a reflecting 



: telescope, such as that used by Dr. Roberts, would result in the 



photographic plate being fogged by the general luminosity of 



; the .sky .sufficiently to obscure dim nebulous streamers and faint 



I stars. With a refracting telescope Dr. Gill has given an ex- 



NO. 1480, VOL. 57] 



