June 15, 1911] 



NATURE 



DOD 



risen to such primary conceptions as that of a " point " 

 and a "line," let alone a "straight line," and whether 

 even the most primitive of the sciences — that of geometry 

 — could have originated, though, perhaps, a sense of geo- 

 metrical form might have been developed through the 

 touch. Be that as it may, however, it is perhaps not too 

 much to say that our knowledge of the extra-terrestrial 

 universe has at least until recently been derived solely 

 through the medium of those a^thereal vibrations which 

 we call light, to which our eyes respond, thus communi- 

 cating with the brain, and though this physical pheno- 

 menon might conceivably have existed apart from the exist- 

 ence of human or other eyes, its existence could scarcely 

 have been recognised, and the science of astronomy, which 

 deals primarily with extra-terrestrial phenomena, could 

 hardly have been. Thus it is that the development of this 

 science has been largely concurrent with the development 

 of those instruments by which the optical efficiency of the 

 eye has been increased. The earliest observations were 

 made with the unaided eye, the results of which have come 

 down to us, being duly recorded either in the form of 

 descriptions or drawings, or even, in some cases, by more 

 or less exact measurements. The introduction of the tele- 

 scope not only immediately opened up fields of vision 

 previously inaccessible, but facilitated in a very marked 

 degree the precision with which measurements could be 

 made, and for about three centuries the telescope has been 

 used as a direct aid to the visual organ. It is improbable 

 that this method of observation will ever be superseded, 

 but there is to-day a growing tendency to replace the 

 human eye at the end of the telescope by the photographic 

 camera. 



The advantages of the method are two-fold : — (i) objects 

 which are too faint to be seen with the eye may yet be 

 photographed by a suflficient extension of the time of ex- 

 posure, and our powers of penetrating the confines of space 

 is thereby increased ; (2) the photographs themselves form 

 a permanent record of what is seen more trustworthy 

 than can be obtained from any drawing or description 

 executed at the hands of man. The early application of 

 the art of photography to astronomy related to the subject 

 from its descriptive aspect rather than to those branches 

 of the science which depend on exact measurement. With 

 the aid of reflecting telescopes of large aperture, it was 

 found possible to delineate with certainty the forms of 

 vast nebulse, so as to establish with great nicety the 

 extension of their convolutions into regions where to the 

 eye alone their traces were quite invisible, even with the 

 aid of powerful telescopes. But, fascinating as these 

 photographs are in aspect, valuable as they are as early 

 records, and suggestive as they are with regard to the 

 cosmical significance of the objects photographed, it can 

 scarcely be maintained that they have so far been prolific in 

 scientific results. Such phenomena as the wonderful spiral 

 forms assumed by many of the nebula;, though proved to 

 be far more common than was previously realised, remain 

 as obscure as ever in their origin. In making this stato- 

 ment, I do not wish to underrate the work of the earlier 

 pioneers in astronomical photography. They have 

 developed methods which we continue to use, and have 

 but little to improve on, and they are not to be blamed if 

 the full value of their work has not been attained in their 

 own generation. Visual work had already been carried so 

 far that the extended vision afforded by photography could 

 hardly have been expected to lead instantaneously to any 

 new and startling phenomenon which should revolutionise 

 existing ideas of the extra-terrestrial universe. It is only 

 as time lapses, and we are able to study the changes which 

 are taking place since these photographs were secured — 

 changes which, as a rule, take place so exceedingly slowly 

 as not yet to be fully established^that tho full advantage 

 of the new method over the old will be realised. 



The enrliest attempts at the photography of celestial 

 objects seems to date back so far as the \pnr 1840, when 

 photographs of the moon were obtained by d.iguerreotype 

 processes, but beyond its use for pictorial representation — 

 to which, porhaps, I might add the d-iily photographic 

 record of thf sun's surface, instituted at Greenwich in 

 187;;, the unsuccessful attempts bv Pritchard at tli-- 

 instillation of Warren de la Rue to utilise it for the deter- 

 mination of the constant of nutation — for a long time it 



NO. 2172, VOL. 86] 



seems to have been regarded with suspicion for purposes 

 requiring the use of exact measurements. While photo- 

 graphy might supplant the astronomical draughtsman, its 

 limitations were in other respects similar to those affecting 

 the draughtsman's art and skill. The latter aspects of its 

 uses are those which I propose chiefly to deal with to- 

 night, not that I wish to underrate the importance of other 

 applications, but that I prefer rather to confine myself to 

 those branches with which I have myself been more directly 

 concerned. The year 1882 saw the first introduction of 

 astronomical photography into the Cape Observatory. No 

 special appliances for the purposes were at the time 

 included in its equipment, but the unexpected appearance 

 of the great comet of that year demanded that an effort 

 should be made to secure a representation of this comet by 

 photographic methods, which had been vigorously developed 

 elsewhere for such purposes. Accordingly, the services of 

 a local photographer were secured, and in order to obtain 

 a sufficiently long exposure to bring out the faint detail 

 of the comet's tail, his camera was attached to one of the 

 equatorial telescopes of the observatory, so that it might 

 partake of the motion imparted to the telescope, and thus 

 be kept pointed on the comet for a prolonged interval. 

 Valuable photographs showing much detail of the structure 

 of the comet were obtained, but the feature to which 

 particular attention was directed was the large number of 

 stars, some of them extremely faint, which, besides the 

 comet, were all duly recorded on the plate. Moreover, the 

 sharpness of the photographic images suggested that they 

 would permit of very accurate measurement, and that, if 

 only similar definition could be obtained in photographs on 

 a more open scale, it might be possible to replace many of 

 the laborious methods of measurement hitherto conducted 

 by direct visual observations on the sky itself by some- 

 what similar methods applied to the photographs. 



There existed at the time no comprehensive survey of 

 the southern skies. A complete catalogue of the stars in 

 the northern heavens, known as the Bonn " Durch- 

 musterung, " giving the positions and magnitude of all 

 stars down to the ninth visual magnitude, with a precision 

 at least accurate enough to ensure their identification in 

 the sky, had been formed by Argelander. and extended to 

 a portion of the southern hemisphere by his successor, 

 Schonfeld, but the remainder of the heavens from 23° S. 

 declination to the South Pole was still comparatively un- 

 known. The photographs I have above referred to sug- 

 gested a means by which this lacuna might be filled, and 

 no sooner was this idea conceived by Sir David Gil! than 

 steps were taken to put it into active execution. Partly at 

 his own personal expense, and partly by means of assist- 

 ance from the Government Grant Fund, administered by 

 the Royal Society of London, the necessary equipment and 

 the services of a photographer were secured, and between 

 the years 1885-9 ^ complete series of photographs of the 

 region extending from 18° S. declination to the South Pole 

 was obtained. In order, however, that these photographs 

 might be of value to science, it was necessary that the 

 facts duly recorded on them should bo minutely examined 

 and rendered accessible to astronomers generally, in the 

 form of a catalogue. As more than 400,000 stars wore 

 involved, this in itself was no light task, and might well 

 give rise to the question as to whether the necessary 

 measurements could not be more advantageously made on 

 the sky rather than on the photographic plates by which 

 it was represented. The method which was subsequently 

 adopted for the formation of this catalogue forms sufficient 

 answer to this question. The resources of the observatory 

 were unequal to so large an undertaking, at least without 

 the cessation of a large proportion of its normal work, 

 and it appeared probable that the photographs would have 

 to remain in the record room of the observatory, valuable 

 for consultation regarding specific points which might 

 arise, but that their complete discussion would have to be 

 relegated to a perhaps remote future. The difficulty was 

 at this juncture mot by a generous offer received from 

 Prof. Knpteyn. of Griiningcn. Prof. Kapteyn was him- 

 self an enthusiastic astronomer, eagerly desirous of 

 devoting himself to work of this chnrnrter, with full 

 • ippreciation of its value, but unfo-' improvided 



with an observatory equipment. He that, with 



the aid of a comparatively small 1 quipment, 



