448 



NATURE 



[March io, 1^98 



hours. The spiral form is clearly shown, while the extent of 

 the nebula is greatly increased. 



Prof. E. E. Barnard, with the 6-inch Willard lens of 30 inches 

 focus at the Lick Observatory, has enormously extended our 

 knowledge of these great diffuse nebulosities. It is quite impos- 

 sible in the limits of this article to deal with these in detail, but 

 the extraordinary form of the nebula round 15 Monocerotis, the 

 enormous diffuse nebulosities in the constellation Cepheus and 

 round the Pleiades, the tremendous extensions of the Orion 

 nebuliE shown in his numerous articles in Astronomy and 

 Astrophysics and the Astrophysical Journal since 1893, 

 are all magnificent examples of the use of the portrait lens 

 in photographing nebulae ; and one can have no hesitation 

 in saying that without the portrait lens we should still 

 be in ignorance of many of these wonderful objects. But 

 Barnard has gone beyond the portrait lens, and has used the 

 lens of a cheap oil lantern, the effective aperture of which is 

 about i^ inches, the focal length being 3^ inches, ratio i to 2 "3. 

 This gives a field of 30 degrees practically flat, the scale of the 

 photographs being 10^3 degrees to i inch on the plate. Twenty 

 photographs in October 1894 {Astronomy and Astrophysics, vol. 

 xiii. p. 811) fully broiight out the value of this instrument. One 

 hour's exposure gave all the Andromeda nebula ;"thirty minutes 

 gave all the diffuse nebulosity round the Pleiades photographed 

 by Archenhold in four hours, and by Barnard with the Willard 

 lens in three hours. The most valuable of all results, however, 

 were those with Orion, obtained on October 3 and 28, 1894, 

 with exposures of 2h. and ih. 15m. The extensive spiral 

 detected by Pickering in 1889 is fully shown in correct propor- 

 tion, and "no description can give any idea of the form and 

 magnitude of this nebula." Extending over 17 degrees in 

 length and nearly the same in breadth it includes almost all the 

 stars of the constellation, and forms iq fact a robe for the body 

 of the giant. The well-known "great nebula of Orion" is but 

 ^ pigmy, compared with the greater nebula revealed by Prof. 

 Barnard's plates, and it is not too much to believe that longer 

 exposure will probably fill the whole constellation with nebu- 

 ' losity, and show that the great nebula is simply the inner 

 terinination and the brightest part of the enormous spiral. 

 ' An English amateur, Dr. E. M. Sheldon {Journal of the 

 British Astronomical Association, vol. v. p. 397), using a lantern 

 lens similar to that used by Prof. Barnard, photographed this 

 enormous spiral in Orion with \\ hours' exposure, in February 

 *i39S. Four hours on the constellation Cygnus with this lens 

 ' gave all the nebulae on Wolf's photograph taken with 13 hours' 

 exposure. 



The nebulosities in the Pleiades have attracted great attention 

 since they were first photographed by the Brothers Henry at 

 Paris in I 1885. These nebulae have always been remarkable 

 from .their intimate relations with individual stars in the cluster 

 — " Maia is a diamond clasp on a curving plume, Electra extends 

 a tentacle towards Alcyone, while Merope has a sweeping gauze 

 trail and probably a nebulous satellite." In striking contrast 

 to this we usually have in other regions of the sky stars and 

 nebulae intimately mixed, although frequently on recent photo- 

 graphs wisps of nebulae are found joining stars, so that the 

 structures appear to resemble festoons of pearls on a gauzy 

 string. The most recent photographs of the Pleiades by Barnard 

 at the Lick Observatory, taken loh. 15m. exposure with the 

 ■XVillard (6-inch) lens ; by Mr. H. C. Wilson, with a similar lens 

 and II hours' exposure ; and by Dr. Max Wolf, have revealed 

 an enormous extension of the Pleiades nebulosity. The whole 

 area is now i "58 square degrees, and there are indications that 

 even 'this is not the real limit, and that more prolonged ex- 

 posures will give still greater extension, probably joining up the 

 whole of the nebulosity into an enormous spiral similar to that 

 covering the constellation Orion. 



Other photographs exhibiting the same class of structure have 

 been obtained of the region round Antares with ']\ hours' 

 exposure by Prof. Barnard at the Lick Observatory. At first 

 sight this new nebulous mass would easily be mistaken for the 

 Pleiades Nebula, and it is a remarkable and very significant fact 

 that both these masses and all other great nebulosities in the 

 Milky Way either occupy vacancies amongist the stars, or are on 

 the edges of such vacancies ; and that in their immediate neigh- 

 bourhood the stars exhibit long vacant lanes and other remark- 

 able features, indicating that the nebula, stars, and vacant lanes 

 are but different features of some vast and at present imperfectly 

 comprehended system of celestial grouping. 



The first lesults obtained by Prof. Bailey, at Arequipa, with 



the Bruce photographic telescope of the Harvard College Ob- 

 servatory have lately been recorded. This portrait lens, the 

 largest in the world at present, has an aperture of 24 inches 

 and a focal length of 135 inches, .so that while the scale of the 

 photographs is equal to that of the international star charts 

 (I minute of arc to i millimetre), the light-gathering power of 

 the telescope is three times as great, and exposures with this 

 instrument need be only about one-third of those required with 

 the standard international telescopes to achieve the same results. 

 But the Bruce - telescope has a further advantage over the 

 standard instruments. Its effective field is 25 square degrees 

 (14-inch by 7-inch plates are used), whereas the effective field of 

 the international instruments is only 4 square degrees in area. 

 The daring experiment of Prof. Pickering in devising, and Mr. 

 Alvan Clark in constructing, this enormous portrait lens has 

 been completely successful (although several eminent astronomers 

 on this side of the Atlantic doubted whether such an instrument 

 could be constructed), and as a result we have an instrument 

 which can do. all the international work on less than 4000 plates 

 and with very much reduced exposure. Prof. Pickering does 

 not at present intend to duplicate the work of constructing the 

 photographic' chart of the stars, but will confine the instrument 

 to nebula; and special regions of the sky, and, with the aid of a 

 24-inch object prism, to spectrum photography. The published 

 preliminary results are of very great value. 



This article ought not to be concluded without mention of the 

 fact that more than one astronomical photographer' is of the 

 opinion that some of the nebujosity shown upon pictures obtained 

 with small portrait . lenses is not realj but due to diffused star- 

 light. A warm controversy has taken place with reference to 

 this point, but' this is, not the place to present the views 

 of the two parties. It has been shown in this article that 

 large instruments, such; as those used for the International 

 Chart, with long focal length but restricted fields, can give us 

 pictures full of delicate details of bright iiebulae, and these 

 photographs are of extreme value ; but we must look to the 

 portrait lens for the larger details and for the fainter nebulosities 

 which are absolutely beyond the reach of any photographic 

 object-glass or mirror. There can be no rivalry between the 

 two classes of instruments ; each is perfect in its way, each will 

 mislead if solely relied upon. Photographs of the same nebulae, 

 both with long focus object-glasses or mirrors and with portrait 

 lenses, are necessary, and must be used to supplement each 

 other, if we are to get correct'ideas of the .phenomena of stellar 

 distribution and the connections between nebulae and stars. The 

 " best instrument- to use "- is not a matter of personal experience 

 nor of individual' opinion :, the optical- and' photographic laws 

 bearing on the subject are well known, and the practical limits 

 of atmospheric definition and instrumental construction are 

 within sight. The ideal instrument for photographing nebulae will 

 probably combine large aperture, short focal length, and the large 

 flat field of the. portrait lens ; will be, in fact, a glorified portrait 

 lens : there are optical reasons why neither the object-glass nor 

 the mirror can be wholly satisfactory. While waiting for this 

 instrument, every possessor of an ordinary rectilinear lens with 

 an ordinary camera can, by strapping his camera on to an equa- 

 torially mounted telescope and Using infinite patience, materially 

 advance our knowledge of nebulae by means of photography. 

 4 . . . R. A. Gregory. 



photo-micrography with 

 powers: 



HIGH 



NO. 1480, VOL. 57] 



PHOTOMICROGRAPHY has for some years past advanced 

 but slowly, although its present status as a means of de- 

 lineating minute structure is undoubtedly much higher than it 

 has ever been." In optical appliances the improvements have 

 been many, the most notable being the introduction of apochro- 

 matic objectives. Their, greater, aperture and freedom from 

 effects of the secondary spectrum have combined to render it 

 possible to obtain good results with much greater ease than 

 formerly. Some of the photomicrographs obtained, however, 

 in the early days of microscopy are even now hardly excelled, 

 although they were produced at the cost of enormous labour, 

 and required extraordinary skill on the part of the operator, with 

 the apparatus then available. The production of satisfactory 

 photographs, when the magnification exceeds one thousand 

 diameters, has always been a matter of some difficulty. One of 

 the greatest of these has been the want of a .source of illumina- 



