May 1, 1891.] 



KNOWLEDGE 



93 



which corresponds to the catalogue and map of M. Wolf, 

 1,421 stars in 1885, and they have since brought the 

 number up to 2,326 by exposures of four hours made in 

 November and December 1887. A difference of only 

 twelve and a half magnitudes of the photometric scale 

 implies that the brighter stars give 100,000 times as much 

 light as the fainter. Mere differences of brightness in 

 the stars composing two streams amounting to 12 or 14 

 magnitudes must not, therefore, be taken as reliable 

 evidence of a difference of distance, if other facts point to 

 an association or probable connection between the two 

 streams. 



No one who examines the accompanying plate will feel 

 inclined to doubt that the nebulous matter shown must 

 be associated with the large stars of the group, and that 

 the streaks of nebulous light are associated with the 

 small stars through which they pass. It has long been 

 recognized that the large and irregular nebulse, as well as 

 the great clusters, are associated with the Milky Way, 

 and are grouped in its neighbourhood ; and one can hardly 

 doubt that the nebulous matter associated with so many 

 stars of the Orion group is intimately associated with the 

 main stream of the Milky Way. 



Dr. Boeddicker, in his large drawing of the Milky Way 

 made at Parsonstown, gives a stream of nebulous light 

 extending from the mam body of the Milky Way, and in- 

 cluding the brighter stars of Orion, a view to which proba- 

 bility is lent by the form and position of the numerous 

 large nebulae in this constellation catalogued by Dr. Dreyer 

 and Prof. Pickering. One of these large nebulie, stream- 

 ing southward for about 60' from the bright star f Orinnis, 

 has recently been well photographed by Dr. Max Wolf of 

 Heidleburg, though it had previously not been overlooked 

 by Dreyer, and had been photographed by the Brothers 

 Henry"' as well as by Prof. Pickering. A strange nebulous 

 line of light in this Orion region, reminding one of the 

 nebulous lines joining stars in the Pleiades group, has 

 been photographed at Harvard. It extends from D.M. 

 -8° 1119 in R.A. 5h. 19m. 0., Dec.-8° 40' to D.M. -8° 

 1132 in R.A. 5h. 21m. 6s., Dec — 8' 50', and passes through 

 about sixteen faint stars, t Other lines of stars are also 

 strikingly evident in this constellation. I have already, 

 in the March number of Knowledge, referred to the re- 

 markable symmetry of the curvature of the branching 

 structures in the great Orion Nebula, with respect to an 

 axis perpendicular to the general plane of the Milky Way. 

 These facts leave very little doubt in my mmd as to the 

 existence of an intimate connection between the nebulous 

 matter associated with the bright stars of the Orion group 

 and the great stream of the Milky Way, and I am con- 

 sequently led to conclude that the girdle of bright stars 

 in which the Orion group and the Pleiades lay, is not 

 widely separated from the stream of smaller stars forming 

 the Galaxy, and that in any case the one cannot be so 

 mucli nearer to us than the other, that the difference of 

 apparent magnitude of the stars in the two streams 

 may be accounted for by the difference of the distance of 

 the streams. 



It does not necessarily follow from the large parallax 

 which has been found for a Ccntdiin that it cannot belong 

 to the stream of large stars referred to, none of which 

 have been found to exhibit any similarly large annual shift ; 

 for the modern parallaxes, which art' chirlly or entirely 

 relied upon, are all relative and not absolute parallaxes ; 

 that is, they arc determined from measures made with 

 respect to adjacent small stars. If the adjacent stars 



* Soo liappurt Annual sur I'Etat de la Observatoire de Paris pour 

 rAiincc 18S7. 



t iioo Annals of Harvard Colleyc Observatorij, vol. xviii., p. 115. 



belong to a stream which is in near proximity to the 

 large stars, as compared with their distance from the 

 earth, we should not expect to find any relative shift, 

 while here and there a large star might be seen projected 

 near to much more distant stars. On the other hand, 

 probably, many wiU prefer to assume that a Ccntauii only 

 chances to lie in a zone of brilliant stars, the rest of which 

 are much more distant, and consequently much larger or 

 brighter, than the star which we now recognize as our 

 nearest neighbour. 



The plate illustrating this paper has been made from a 

 photograph kindly given to me for the purpose by Mr. 

 Isaac Roberts. It will be noticed that the direction of the 

 upper streak of nebulous light is continued by a line of 

 stars which extends nearly all across the plate, and that 

 there is a somewhat similar line of stars extending from 

 Plfinne (the bright star just to the north of Atlas ') nQa.v\y 

 across the plate just to the north of Alci/mu- and Ehxtra. 

 The eight rays crossing the brighter stars, and making 

 them appear like the conventional symbols for stars on a 

 star map, are due to a diffraction effect caused by the 

 arms supporting Mr. Isaac Roberts's camera in the focus of 

 his mirror. 



Hcttcrs. 



[Tho Editor does not hold himself responsible for the opinions or 

 statements of correspondents.] 



ON THE COMPARISON OF THE PHOTOGRAPHS OF THE 

 MILKY WAY IN a=17n. 5Gm. 8= -2S° IN ■' KNOWLEDGE" 

 FOR JULY IS'JO, AND MARCH 1891. 



To the Editor of Knowledge. 



De.\k Sir, — I have read with much interest Mr. Ran- 

 yard's article In Knowxedge for March 1891, about the 

 photographs of the Milky Way in 17h. 56m., —28°, made 

 by me in 1889, and by Mr. Russell m 1890. 



I have no hesitation in attributing the difference 

 between these pictures (taken with instruments so similar) . 

 entirely to the development of the negative. As I have 

 taken occasion to remark elsewhere (Pub. A.S.P. II. 10), 

 the utmost care must be exercised in the development of 

 the Milky Way pictures to bring out the cloud forms 

 clearly and strongly. 



It will be seen that Mr. Russell's photograph was given 

 one hour and twenty minutes longer exposure than mine, 

 and with a zenith distance on the meridian of only 6°, 

 while the zenith distance here was 65°. The plates used 

 for the Lick Observatory pictures were Seed, sensitometer 

 No. 26. They are among the best that are made. I 

 have examined Mr. Russell's photograph and can find no 

 evidence of change in the cloud forms of the Milky Way 

 as shown on his picture and mine. It is an extremely 

 uncertain thing to compare two photographs like these, 

 where one shows only feebly the cloud forms and the 

 other shows them strongly and conspicuously. An exact 

 comparison, however, may be made by densely over- 

 printing the picture made here, so that the cloud forms 

 will become as feebly shown as are those in the Sydney 

 picture. Such a comparison I have made. The two 

 photographs are identical in every respect, except that 

 there are apparently several large stars near the middle 

 of Mr. Russell's picture which are not shown on mine, 

 and which are undoubtedly defects in his plate. 



I forward to you the photograph which I have used for 

 comparison with Mr. Russell's. You will see the two are 



* For the names of tho Pleiades stars, see the diagram in Kkow- 

 LEUOE for January 1889. 



