April 1, 1893.] 



KNOWLEDGE. 



69 



" THE V ARGUS NEBULA. 



By A. C. R.4NYARD. 



WE are indebted to Dr. Gill for the very beautiful 

 photograph of this nebula, which we are 

 enabled to present this month to our readers, 

 for comparison with the photographs of Mr. 

 Russell published in last mouth's number 

 of Knowledge, and with the drawing of the nebula 

 laboriously made by Sir John Herschel during his visit to 

 the Cape in 1834-38. 



On first comparing the drawing with the photographs, 

 probably most of our readers will fail to recognize much 

 similarity ; but, on closer inspection, many of the striking 

 features of the nebula as shown in the photographs will be 

 recognized as having certainly been seen by Hir John 

 Herschel. For example, the dark oval space, shaped like a 

 "kidney bean, "shown in the lowerpartof Sir John Ilerschel's 

 drawing, to the right of the centre, is distinctly recognizable 

 on Dr. GUI's photograph. It is to be found in the hori- 

 zontal rows of squares marked out by the reticule which 

 have been numbered 5 and 0, and in the vertical column of 

 squares marked 26. That this is the dark patch which 

 was noted and attempted to be drawn by Sir John 

 Herschel is evident from the stars which surround the 

 dark region. The positions of these stars were careful!}- 

 measured, and their magnitudes were noted and repeatedly 

 compared by Sir John Herschel. Two of the biightest of 

 them are on the south following side of the dark patch, 

 and lie just upon the edge of the nebulosity. There 

 is also a line of stars which lie upon the preceding side of 

 the dark patch, and seem to coincide with and mark out 

 the limit of nebulosity. The brightest of the stars on the 

 preceding side of the dark area is at its northern end, and 

 lies just within the nebulosity. 



It will be noticed that these stars, though they agree 

 satisfactorily in position with the stars given in Sir John 

 Herschel's drawings, differ so materially in their relative 

 magnitudes that we are forced to conclude either that 

 many of the stars are variables, or that the photographic 

 magnitudes (which agree on comparing Dr. Gill's photo- 

 graph with Mr. Russell's) do not at all correspond with the 

 visual magnitudes. A similar remark applies to the four 

 stars shown in Sir John Herschel's drawing on the 

 following side of the pair of stars bordering the south 

 foUowiug edge of the dark oval area. 



Another feature of Sir John Herschel's drawing of the 

 nebula, which is distinctly recognizable in the photographs, 

 is a narrow dark channel which runs north and south near 

 to the centre of the lower (or southern) part of Sir John 

 Herschel's drawing. This will be found in Dr. Gill's 

 photograph on the preceding side of the vertical column of 

 squares numbered 23, and extending from the horizontal 

 row numbered 7 to horizontal row 5. The stars at the 

 southern end of this dark channel on the Herschel drawing 

 can, with difficulty, be recognized on the photograph, but 

 a group of stars forming two closed curves on the north 

 following side of the dark channel can be distinctly 

 recognized on the photograph, in columns 22 and 23, row 8. 

 In this region the nebulosity, as drawn by Sir John 

 Herschel, entirely differs from the photographs. 



One of the most striking features of Sir John Herschel's 

 drawings, which is not shown in the photographs, is a 

 curious trident-shaped structure on the following side of 

 the centre of the Herschel drawing. The upper part of 

 this structure has been compared by Dr. GiU to the figure 

 of a swan, but although it is one of the brightest structures 

 in the lower half of Sir John Herschel's drawing, there 

 seems to be no nebulosity corresponding with it in position 



on the photographs which, on the hypothesis that there 

 has been no change in the nebula since 1834, could have 

 been represented by Sir John Herschel by this swan- 

 shaped structure. Its place on Dr. GiU's and Mr. Russell's 

 large photograph is nearly free from all nebulosity — and 

 even on Mr. Russell's very dense photograph taken with his 

 Dalmeyer C-inch lens, which was reproduced in the last 

 number of Knowledge ; though there is a general faint 

 nebulosity in the corresponding region, there is no trace of 

 the curving form drawn by Sir John Herschel. 



The question whether we are warranted in assuming 

 that a change of such magnitude has taken place in the 

 nebula in a period of less than sixty years is one of great 

 interest. The structure we are considering measures about 

 seven minutes across its smaller diameter, and even if we 

 assume that the nebula is not at a greater distance from us 

 than our nearest neighbour amongst the stars (a Centauri), 

 the diameter of the structure would be equivalent to 280 

 times the diameter of the earth's orbit, or to more than 

 nine times the diameter of the orbit of Neptune ; con- 

 sequently, the disappearance of such a structirre would 

 involve either a change of brightness of the matter 

 distributed over an enormous space, or a very rapid 

 motion of the nebulous matter. The swiftest-moving star we 

 know (1830 Groombridge) would only move across a space 

 equal to the diameter of the missing structure in sixty years. 



A series of observers have examined the nebula since Sir 

 John Herschel's drawing was published, and have noted 

 changes which they believed could not be explained by in- 

 accurate draughtsmanship, but it seems to me that their 

 drawings differ as much amongst themselves as Sir John 

 Herschel's drawing difl'ers from the photographs. The 

 difficulty of accurately drawing an object like a nebula, 

 which cannot be sketched with definite outUnes, can only 

 be reahzed by those who have made an attempt to repre- 

 sent a nebula or a corona or other object where the likeness 

 or want of similarity depends upon the due graduation of 

 tints, and where an alteration in the intensity of the 

 brightness of any area frequently involves a corresponding 

 alteration in the tints representing the brightness of sur- 

 rounding areas. In a question of such importance I prefer 

 to wait until we have the unbiassed evidence of i^hotographic 

 plates taken at long intervals, before admitting that there 

 is undoubted evidence of changes rapidly taldug place on so 

 gigantic a scale. 



The following list of papers on this nebula may interest 

 those who desire to examine more closely the evidence we 

 already possess with regard to changes which most of 

 the observers who have given their attention to the nebula 

 believe are rapidly taking place within it. I am indebted 

 to my friend Mr. Sadler for several of these references. 



1834-1838. — Sir John Hi>rschel, " Cape Observations," pp. 32-17. 



1861-1864.— Mr. F. Abbott, of Hobart Town, Tasmania, Monthlif 

 Notices^ xxiT., pp. 2-5. 



1863-1 868.— Mr. F. Abbott, MoiMIy Xotices, xxriii., pp. 200-202. 



1870-1871.— Mr. P. Abbott, Monthly :![oiices, xxix., pp. 226-234. 



1871. — Captain J. F. AV. Herschel, JSIonihly Xotices, xxix., p. 235. 



1854-1S70.— Mr. J. Tebbutt, Monthlii Xotices, xxiv., p. 211. 



1871. — Mr. H. C. Rnssell, "' Transactions of the Eoval Society of Xe« 

 Sontli Wales for 1871," pp. 1.5-24.' 



1869-1871. — Mr. P. Macgeorge, " Transactions of the Royal Society 

 of Tictoria," Tol. x., pp. 106-113. [This paper con- 

 tains drawings by Mr. Le Sueur, as well as by Mr. 

 Macgeorge.] 



1882. — Mr. C. E. Peek, " Rousdon Observatory Observations for 

 1882-5." [Contains a drawing of the nebula, made by 

 Mr. Peek at Jimboui', Queensland.] 



1891.— Mr. H. C. Russell, Monthly Xotices, li., pp. 494-497. 



Most of these observers have given special attention to 

 the region in the northern nebulous mass around the dark 

 opening which, in Sir John Herschel's drawing, looks like 

 a keyhole. He refers to it ia the " Cape Observations," 



