March io, 1898] 



NATURE 



447 



The Portrait Lens in Nebular Photography. 

 It was at one time supposed that large telescopes were 

 necessary to obtain valuable pictures of celestial objects ; but as 

 the work went on, it was found that ungainly instruments were 

 not at all essential, and that excellent results were given by 

 instruments of very moderate dimensions. For photographing 

 faint stars, where the images on the photographic plates are 

 practically points, the aperture of the object-glass or mirror is 

 almost the only factor to be taken into account in estimating 

 efficiency ; but for nebulce, comets and similar celestial objects, 

 the images of which cover sensible areas on the plates, the 

 ratio of focal length to aperture is all-important, and the actual 

 aperture is a secondary consideration. 



Experience has shown that a lens constructed in the same 

 way as a portrait lens for use in an ordinary camera, is really 

 the best instrument for several branches of celestial photography. 

 The lens must of course be mounted, so that it can be made to 

 follow the motions of the celestial sphere, but in other respects 

 the camera need not differ essentially from that used in the 

 ordinary portrait studio. The advantage of the portrait 

 lens over the photographic telescope is that the field of view 

 is much greater — it is therefore able to take a broader view 

 of things. 



The telescopes used for the international star catalogue and 

 chart may be taken as the standard instruments for star work. 

 The aperture of the object glass is 13 inches, and the focal length, 

 roughly, 130 inches (ratio i to 10), the effective field being a 

 little over 2 degrees square (4 square degrees on the celestial 

 sphere). A portrait lens of 6 inches aperture and 30 inches 

 focus (the favourite size with those who use portrait lenses for 

 celestial photography) has a ratio of aperture to focal length of 

 1 to 5, and an effective field of 15 degrees square (225 square 

 degrees). In photographing a nebula, this portrait lens only 

 requires about one-half the exposure necessary when the standard 

 telescope is used, while the area covered is nearly forty times as 

 great. It follows, therefore, that for an extended nebula covering 

 over 100 square degrees on the celestial sphere —and such 

 nebulae exist, and have been photographed — the portrait lens can 

 give in two hours a complete picture comparable, in area of sky 

 covered, with a picture that would require twenty- five exposures 

 <?f four hours each with the standard star telescope ; and that, 

 whereas the picture with the portrait lens would all be on one 

 plate and taken with one exposure, that with the standard 

 telescope would be a jnosaic built up from at at least twenty-five 

 plates taken at different times, and consequently under very 

 various atmospheric and astronomical conditions. 

 : During the past eight years or so, a large number of photo- 

 graphs of nebula; and nebulosity have been obtained by means 

 of portrait lenses ; and year by year the work done, both in this 

 and other branches of astronomical photography, has been admir- 

 ably summarised by Mr. Albert Taylor in the Photography 

 Annual. Every one interested in the progress of celestial 

 portraiture should refer to these annual records of results, as we 

 have had frequently to do in preparing the subjoined account 

 of the use of the portrait lens in astronomical photography. 



The great advantage of the portrait lens or doublet over the 

 telescope was admirably brought out by Prof. W H. Pickering, 

 who, using a small camera at Wilson's Peak, California, in 1889, 

 obtained a photograph including on one plate the whole con- 

 stellation of Orion. The great nebula in Orion was clearly 

 shown, but the chief value of the photograph lies in the large 

 number of detached nebulous patches shown, which appear to 

 be part of an enormous zone of nebulosity encircling the great 

 nebula, and practically covering the entire constellation. The 

 ^.\istence of this great mass of nebulous matter, and its obvious 

 connection with the well-known nebula in the sword-handle of 

 Orion, would probably never even have been discovered with 

 long-focus instruments with small photographic fields ; yet with 

 the portrait lens it is a comparatively easy object. 



In 1890, Mr. H. C Russell, F R.S., obtained a large number 

 of striking photographs of nebula; and the Milky Way with a 

 6-inch Dallmeyer portrait lens attached to the mounting of 

 his telescope. The pictures thus obtained show many details 

 of structure which are quite invisible in the telescope. Ex- 

 posures of between four and five hours brought to light many 

 peculiarities of arrangement of stars in Nebecula Major and 

 Minor — the Magellanic clouds — and showed that the whole of 

 the former apparently detached portion of the Milky Way had 

 the structure of a complex spiral nebula with two nuclei 

 two degrees apart. The Nebecula Minor as portrayed by Mr. 



NO. 1480. VOL 57] 



Russell's camera bears a resemblance to the well-known 

 Dumb-bell nebula. 



Following closely on Prof. W. H. Pickering's work in 1889, 

 the greatest advances in the photography of nebula? in 1891 

 were due to Prof. Barnard at the Lick Observatory, Dr. Max 

 Wolf at Heidelberg, Mr. Russell at Sydney, and Dr. Archen- 

 hold at Helensec, each of these observers using ordinary 

 portrait lenses. 



Dr. Max Wolf, with a 2j-inch aplanatic lens of 7f inches 

 focus, photographed in one hour all the nebulae in the 

 Pleiades obtained by the Brothers Henry at Paris with four 

 hours' exposure with the 13-inch charting telescope ; and after- 

 wards using a 4-inch Vbigtlander Euryscopic lens and a 5-inch 

 Kranz Euroscopic lens. Wolf obtained more extension of 

 the great nebula in Orion in 4 hours than Dr. Roberts had 

 obtained with 4^ hours' exposure with his 20-inch mirror. 

 These same plates of Wolf amply confirmed Prof. W. H. 

 Pickering's results, and revealed an enormous amount of new 

 detail in the nebula around ^ Orionis. 



Turning his attention to the constellation Cygnus on June I, 

 1891, Dr. Wolf, by 3 hours' exposure with the 5-inch Kranz lens, 

 discovered an enormous nebula full of the most complex struc- 

 ture, and connecting a number of bright stars with many fainter 

 ones in the Milky Way (Astr. JVach. , ^048). Starting apparently 

 from a central point, the nebula spreads out branches which 

 curve and meet ' ' fold on fold of nebulous matter surging over 

 the sky" and " becoming notably compact and luminous in the 

 immediate neighbourhood of 7 Cygni." Some of these branches 

 are 8° long, so that at least four plates would be required fox one 

 of them with a photographic telescope of standard size ; but the 

 portrait lens secures all on one plate, and their true relations to 

 each other and to the involved stars become at once apparent. 



The great feature of this photograph, and of many subsequent 

 pictures, is the obvious connection between bright stars and 

 fainter ones. It was formerly believed that the brightness of a 

 star was to some extent a measure of its distance from us, but 

 the photographs with portrait lenses effectively disposed of this 

 idea. Stars of nearly first magnitude were found to be joined 

 to stars of the eighteenth magnitude by wisps of nebula, and to 

 obviously form part of the same system in the heavens. These 

 stars, the brightest of which are about 4,000,000 times more 

 luminous than, the faintest, must be at practically the same 

 distance from us, and their differences in brightness must be due 

 to differences of rsize .or physical condition, or both, and not to 

 any differences in distance. 



Another interesting result from the photographs was the 

 proof of the connection of nebulae with stars of the Wolf-Rayet 

 type of spectrum — stars with bright lines in their spectra — which 

 Sir Norman Lockyer classifies as stars only just condensed from 

 nebulae, and next in order of evolution. Dr. Roberts, with 

 3i hours' exposure with the 20 inch mirror, could detect no 

 nebulosity round these stars in Cygnus, but the portrait lenses 

 with exposures extending up to 13 J hours, indicated that all 

 these stars are nebulous. 



This work of Wolf was supplemented by some splendid results 

 obtained by Mr. Russell at Sydney Observatory about the same 

 time. The lens used, a 6-inch Dallmeyer of 30 inches focus, 

 gave, with three hours, as much extension of the great nebula 

 round the star rj Argus as the 13-inch charting telescope showed 

 with twice that exposure, while the relations of the nebula to 

 the surrounding stars were much better shown. Two long ex- 

 posures with the same lens on those curious detached portions 

 of the Milky Way in the southern heavens, known as the 

 Nebeculae Major and Minor, revealed a great amount of pre- 

 viously unknown nebulous matter, much of which would be very 

 difficult to obtain with larger instruments. 



Dr. Archenhold, at Helensec, recognising the true principles 

 of photography of celestial objects of large size, had two lenses 

 constructed by Busch, the ratio of aperture to focal length being 

 I to 4*5 and 1 to 2 '5 respectively. With these lenses he photo- 

 graphed a very striking and extensive nebula near | Persei on 

 October 27, 1891, with 32J minutes' exposure. This new nebula 

 resembles in many respects the great nebula in Andromeda, but 

 probably covers a much greater area of the sky. The great 

 effectiveness of this special lens is shown by eye observations 

 with a 12-inch telescope; for even with the photograph to 

 indicate where to look for details, scarcely any trace of the 

 nebula can be made out (Astr. Nah., 3082). Dr. Scheiner, at 

 Potsdam (Astr. Nach., 3157), has photographed this great 

 nebula with a 4-inch portrait lens, giving exposures up to six 



