March io, 1898] 



NATURE 



443 



THE PHOTOGRAPHY OF NEBULA... 



"PHOTOGRAPHY has been a helpful handmaid in many 

 branches of astronomical science, but in no department is 

 the value of her assistance more cle&rly seen than in that which 

 is concerned with the forms and structures of nebulce. This is 

 hardly to be wondered at when the fine texture, the almost im- 

 perceptible gradations of light, the intricacy of detail, and the 

 vai-iety of nebulous forms are considered. Many vigils must be 

 kept by an astronomer before the trend and comparative dis- 

 tinctness of a particular nebulous feature is satisfactorily 

 observed ; and even where this has been done, to represent the 

 characteristic faithfully is beyond the power of any but the most 

 accomplished draughtsmen. The unimaginative photographic 

 plate, however, looks heavenwards for a few minutes and has 

 imprinted upon it not only the delicate details which tease the 

 eye of the observer and elude the skill of the artist, but also 

 records a greater extent of celestial mist than the human eye is 

 capable of grasping. On this account exceptional interest is 

 attached to what has been accomplished in the portraiture of 

 nebulae, and the following survey of the subject will serve to 

 show some of the roads along which progress has been made. 



Early Days of Nebular PhotcJgraphy. 



Dr. Henry Draper was the pioneer of nebulaf photography ; 

 he succeeded in obtaining a photograph of the nebula of Orion 

 on September 30, 1880. ("Washington Observations," vol. 

 XXV., 1878. Appendix i. p. 226.) Only the brightest parts of 

 the nebula were comprised within the picture ; nevertheless, the 

 result was such as to show that photography had great possi- 

 bilities before it as a delineator of nebula;. Encouraged by the 

 tangible fruits of his labour, Dr. Draper took a numljer of 

 photographs of Orion's nebula, and in March 1881 obtained a 

 picture showing stars fainter than the fourteenth magnitude ; that 

 is, stars only just within the limits of visibility of the telescope 

 employed in the work. This fact was not lost upon him, for in 

 a short note communicated to the Paris Academy of Sciences in 

 April 1881 (Comptes rendus, vol. xcii. p. 964, 1881), he re- 

 marked that astronomers might reasonably expect to photograph 

 stars which were quite beyond the visual reach of the most per- 

 spicuous observer ; in other words, that a sensitive plate at the 

 eye-end of a telescope could see objects which were too faint to 

 produce any impression upon the retina of an observer using the 

 same instrument. The picture which led to this remark was 

 taken with an exposure of 104 minutes. Towards the begin- 

 ning of the following year, a fine negative was produced by ex- j 

 posing a gelatino- bromide plate to the nebula for 137 minutes, i 

 This photograph comprised more of the nebulous matter, and ! 

 especially of the delicate outlying parts, than any of the previous 

 ones. In commenting at the time upon the strikingly perfect i 

 representation of the nebula afforded by the picture (" Washing- 

 ton Observations," vol. xxv. ; Appendix i. p. 227), Prof, E. S. 

 Holden compared it with Bond's drawing of the same object, j 

 This observer spent several years scrutinising the nebula, and, as ; 

 a result of his patient observation, was able to produce a picture 

 which represented its features with greater accuracy and artistic 

 effect than had previously been attained. Dr. Draper's photo- . 

 graph of the nebula was taken in a little over two hours, yet 

 Prof. Holden confessed that for nearly every purpose it was in- 

 comparably better than Bond's hand-drawn picture. It was 

 evident from this that a new epoch of nebular observation had 

 been opened. Exact and automatic representations of nebulae 

 were to take the place of the strange, and often crude, drawings 

 of these objects. The new method inaugurated by Dr. Draper 

 has developed so much that, at the prese;it time, it may almost be 

 said that photography entirely holds the field as a nebula-artist. 



A nebula rarely hasa definite form, like the sun and moon. It 

 presents the appearance of a cloud having more or less irregular 

 outlines, and of which tlje various parts differ greatly in bright- 

 ness. It results from this that photographs of the same nebula 

 may be very different in appearance, for their characters depend 

 upon the power of the telescope employed in their production, 

 the lime during which the sensitive plate was exposed, the 

 sensitiveness of the plate, the transparency of the atmosphere, 

 and many other causes. While Dr. Draper was working upon '• 

 the Orion nebula in America, Dr. Janssen was experimenting j 

 at Meudon with a view of determining the influence of some of { 

 the variable conditions upon the results obtained (Co}nptes , 

 rendtts, vol. xcii. p. 261, 1881). By taking photographs with I 

 exposures of five, ten, and fifteen minutes respectively, the 



NO. 1480, VOL. 57] ' 



eminent French investigator found that the longer the nebulous- 

 light was beating upon the sensitive film, the greater was the 

 extent of nebulosity portrayed. It Was this fact which per- 

 mitted Dr. Draper to obtain his epoch-making picture, and has 

 led to even more remarkable results during the past few years. 



So long ago as 1874, Dr. A. A. Common w4S engaged in 

 celestial photography, but it was not until May 1S82 that he 

 exhibited a photograph of the nebula in Orion (Monthly 

 Notices, R.A.S., vol. xliv, p. 222, 1883-84). The instrument 

 used by him was a reflecting telescope three feet in diameter, 

 specially constructed for photographic work. Such a large 

 instrument is necessarily difficult to adjust and .drive, and a 

 laborious series of experiments had to be made before it could 

 be said to be in working order. But the time spent in devising 

 improvements was well repaid by the photograph of the Orion 

 nebula taken by Dr. Common in January 1883. The photograph 

 showed details of the nebula never before properly represented by 

 the hand, and which can hardly be discerned by the eye. With 

 the confidence that comes from experience, it was then predicted 

 that "we are approaching a time when photography will give 

 us the means of recording in its own inimitable way the shape 

 of the nebula and the relative brightness of the different parts,, 

 in a better manner than the most careful hand-drawings." 



This prophecy was strikingly fulfilled in less than three years 

 after it was made. 



The Pleiades Nebula. 



In the early part of the year 1885 a fine photographic telescope 

 was added to the equipment of the Paris Observatory, and' 

 placed under the control of two brothers, MM. Paul and 

 Prosper Henry. The instrument had only been mounted a few 

 months when it was used to photograph a cluster of stars— the 

 Pleiades — which has attracted attention from time immemorial. 

 The picture obtained showed truthfully the relative positions 

 and grandeurs of the stars in and near the beautiful bunch of 

 lucid points to which the telescope had been directed. But it 

 was not so much the imprints of hundreds of stars that made 

 the picture interesting to astronomers, as the fact that a new 

 nebula appeared upon it. Round "stately Maia" — a star just 

 visible to the naked eye — several wisps of nebulosity were clearly 

 portrayed. Three further photographs of the same celestial region 

 confirmed the existence of this nebulous matter, though no trace 

 of haziness had previously been detected by ordinary telescopic 

 observation (^Monthly Notices, R.A.S., vol. xlvi. p. 98, 1885-86). 



It is a remarkable fact, however, that when an object has been 

 discovered an observer is frequently able to see it, though be 

 may have passed it over many times in previous surveys. So it 

 was with the nebula round Maia. Very shortly after the 

 announcement of the discovery had been made, M. Struve 

 turned the 30-inch refractor at Pulkova towards the star to 

 which attention had been directed, and found that he could 

 distinctly see the nebulous surroundings {Comptes rendus, vol. 

 cii. p. 281, 1886). ■ 



But faint objects are not only overlooked by- the observer 

 while viewing celestial scenery through his "optic tube"; they 

 often go undetected on photographs themselves. The announce- 

 ment of the discovery of the nebula recalled to Prof. E. C. 

 Pickering's mind that certain irregularities had been noticed in. 

 a photograph of the Pleiades taken at Harvard College Observ- 

 atory on November 3, 1885 [Astrononiische Naehrichten, "voX, 

 cxiii. p. 399, 1886), that is, thirteen days before the MM. 

 Henry obtained their first photograph showing Maia's nebulqus^ 

 surroundings. A re-examination of the Harvard College picture 

 confirmed his surmise that the markings, which had previously 

 been passed over as blemishes, were really the wisps of nebulosity 

 photographed at Paris. Extending the scrutiny to the remainder 

 of the Pleiades, indications of nebulous light were found about 

 Merope, and a strange, narrow streak was spen projecting ifrom 

 Electra.. The Paris photographs showed similar appendages ta 

 these stars. This was not, however, a new discovery ; the 

 nebula near Merope was seen by W. Tempel while observing at 

 Venice as far back as 1859 {ibid., vol. liv. p. ?86, 1 861), and 

 though several astronomers unsuccessfully searched for the 

 object, many illustrious observers had attested to its existence 

 {Monthly Notices, R.A.S. ,\\. Tp. 622, 1879-80). Photography 

 established the reality of Tempel's observations ; and what is- 

 more, it was soon able to show that the faint patch, which had 

 been the subject of so much discussion, was but a bright part of 

 a vast nebulosity, in which the clustering stars were immersed. 



Dr. Isaac Roberts was the astronomer who brought to light 



