50 



KNOWLEDGE. 



[March 2, 1891. 



THE MILKY WAY IN THE SOUTHERN 

 HEMISPHERE. 



]')Y A. C. Ranyaku. 



Tl 1 1', plates illustrating this paper have been made 

 from photographs taken by Mr. Russell, Director 

 of the Sydney Observatory, New South Wales. 

 They will bear close examination with a magni- 

 fying glass, and the sharp images of the small 

 stars show how very accurate and steady must have been 

 the motion of Mr. Russell's driving clock, which kept the 

 camera directed to the stars during the exposures by a 

 motion about the polar axis of the instrument in a con- 

 trary direction to the earth's motion about its axis. The 

 driving clock was controlled by an electrical apparatus, 

 contrived by Mr. Russell, which connects it with a govern- 

 ing clock and two heavy pendulums. These photographs 

 form a very satisfactory certificate of Mr. Russell's method 

 of electrical control, and show that the differences of re- 

 fraction due to the changes of the altitude of stars during 

 long exposures may be more accurately compensated for 

 than had hitherto been supposed. 



The scale of these star-pictures will be best appreciated 

 from photograph No. II., which shows the greater part of the 

 lower or southern portion of the constellation of Orion. 

 The three stars at the bottom of the picture form the belt 

 of the constellation giant. The three stars in a nearly 

 vertical line above them, with the Great Nebula about the 

 central star, form the sword ; and the two stars on either 

 hand towards the top of the picture form the feet of the 

 giant. Unfortunately the printers have turned this pic- 

 ture, as well as No. III., with the south point at the top 

 instead of at the bottom of the pictures, as in Nos. I. 

 and IV. 



Picture No. II. enables us to show that the central 

 line of symmetry of the Orion Nebula, towards 

 which the great curving structures springing fi-om the 

 neighbourhood of the trapezium are synclinal, is, as 

 nearly as one can judge, at right angles to the medial 

 plane of the Milky Way. A reference to any good star 

 map or globe showing the Milky Way, will show that the 

 line passing through the stars of Orion's Belt is nearly 

 parallel to the plane of the Milky Way, the axis of sym- 

 metry of the Great Nebula is nearly square to the line of 

 the belt, and consequently at right angles to the general 

 plane of the Milky Way. Another fact worth mentioning, 

 as showing an apparent symmetry with regard to the 

 plane of the Milky Way, is that the two remarkable nebu- 

 lous lines joining stars in the Pleiades Nebula* are approxi- 

 mately parallel to the general plane of the Milky Way, 

 though neither of the lines are quite straight or parallel 

 the one to the other. The edges of the great curving 

 structures of the Orion Nebula are all harder on the 

 inside towards the axis of symmetry which passes through 

 the trapezium, and softer or more nebulous on their outer 

 or convex edges. The scale of picture No. II. is too small 

 to show this, though it is partly shown on the larger 

 picture published in the May number of Kno^\xedge for 

 1889, and referred to in the accompanying article on the 

 Nebula. It is still more evident on the original negatives, 

 and would alone prove, even if we knew nothing of the 

 symmetry of curvature of the tree-like structures which 

 spring from the trapezium region, and of the canopy which 

 overhangs the whole nebula, that there are mighty forces 

 acting towards and away from the axis of symmetry — 

 that is, parallel to the plane of the Milky Way. 



* See Knowledge for January 18S9, pp. 69-70. 



It will be remembered that both the Pleiades Nebula 

 and the Orion Nebula lie a little to the south of 

 the Milky Way, at about the same distance from its 

 medial plane. There seems to be no obvious connec- 

 tion between the plane in which the Great Nebula in 

 Andromeda lies and the plane of the Milky Way, and the 

 same remark applies with regard to the Ring Nebula in 

 Lyra. The elliptic patches of light into which these 

 nebulne project have not their major and minor axes 

 parallel and perpendicular to the medial line of the Milky 

 Way, as would lie the case if the planes of these nebulre 

 were either parallel or perpendicular to the plane of the 

 Milky Way ; but though these two nebulae are both on the 

 borders of the Milky Way, the spiral nebubf do not seem 

 to be associated with the Milky Way in the same intimate 

 manner that the other large and irregular nebulie are ; 

 thus the spiral nebulie in Ursa Major and Canes Venatici 

 are at some distance from the Milky Way, and their 

 planes are evidently not parallel to one another. Even 

 the small elliptic nebube H.V. 18, discovered by Miss 

 Caroline Herschel close to the great Andromeda Nebula, 

 evidently lies in a different plane from the Great Nebula. 



Picture No. I. represents the part of the Milky Way in 

 Sagittarius photographed by Mr. Barnard, which we 

 reproduced in the .July and August numbers of Knowledge. 

 The differences between Mr. Russell's photograph taken 

 at Sydney on the 2nd October 1890, and Mr. Barnard's 

 photograph taken at the Lick Observatory on the 1 st .\ugust 

 1889, are very curious and worthy of close attention. Both 

 photographs were taken with six-inch portrait lenses of 

 about 31 inches focus, so that they are on about the same 

 scale, and the pencil of light falling on the sensitive plate 

 was in each case of about the same intensity. Mr. Barnard's 

 photograph was exposed for .Sh. 7ni., and Mr. Russell's 

 for 4h. 2m. Nevertheless, Mr. Barnard's photograph 

 shows much more of the nebulous structure of this region 

 of the Milky Way than Mr. Russell's. This might be 

 due to dift'erences in the method of development, or to a 

 difi'erence in the sensitiveness of the plates used (Mr. 

 Russell seems to have used Ilford extra lapid plates ; and 

 Mr. Barnard, I believe, used some plates prepared by the 

 American Seed-plate Company). But it is remarkable 

 that the relative brightness of the nebulous areas on 

 difl'erent parts of the plates do not correspond with one 

 another on the Russell and Barnard plates. The reader 

 should refer to the large picture from Mr. Barnard's nega- 

 tive published in the July number of Knowledge in order 

 to follow what I am about to say. He will then see that 

 there are great differences in the brightness of different 

 parts of the nebulous structure, while the relative bright- 

 ness of the stars is much the same on both the plates — 

 with one notable exception, which it may be weU to draw 

 attention to before referring to the difi'erences in the 

 brightness of the nebulous light. 



In Mr. Barnard's picture there are two clusters of stars 

 near to the edge of the field at the bottom of the plate. 

 Only one of these clusters, \-iz. that to the right hand or 

 western side of the field, is shown in Mr. RusseUs plates. 

 The almost equally brilliant cluster near to the bottom of 

 Mr. Barnard's plate is missing in the Sydney photographs, 

 for Mr. Russell has sent over silver prints from two nega- 

 tives, one taken on the 17th September 1890, and the 

 other, reproduced in our picture No. I., taken on the 2nd 

 October 1890. I at first thought it possible that both 

 photographs might by mistake have been copied from the 

 same negative, and that the dift'erences between the Lick 

 and Sydney photographs might possibly be due to in- 

 equahties in the sensitiveness of different parts of the 

 film of Mr. Russell's plates. But it is evident that no 



