May 1, 1896.] 



KNOWLEDGE. 



109 



among several hundreds of stars in each chister one single 

 variable. 



Prof. Bailey's discovery, which has been independently 

 confirmed by Mrs. Fleming, Prof. Pickering, and Prof. 

 Hale, gives most valuable support to my idea ; and that 

 support is the more valuable because the phenomena of 

 variabiUty, when caused (as is mostly the case) by cooling, 

 have much resemblance to the phenomena of Novae. What 

 we want now is the spectrum of such a cluster rich in 

 variables. Possibly it will show us the gaseous lines of 

 its numerous dark stars. 



THE NEBULOSITY ROUND 15 MONOCEROTIS. 



By E. Walter Maunder, F.R.A.S. 



THE original of the accompanying photograph was 

 taken by Mr. E. E. Barnard in the course of a 

 photographic study of certain regions of the Milky 

 Way undertaken by him, at the Lick Observatory, 

 with the Willard lens of 6-inch diameter and 

 31 inches focus. He found the region lying north and 

 east of Orion to be singularly rich in large diffused 

 nebulosities, of which one connected with 15 Monocerotis 

 was most especially noteworthy. Commenting on a plate 

 exposed on January 24tb, 1894, for 2h. 80m., on the field 

 of which J Geminorum is the centre, he points out" that 

 " this group of bright stars is mixed up with a knotted and 

 wispy nebulosity, extending faintly and irregularly north 

 and westerly to an extent of nearly two degrees. The 

 nebula is irregular in outline, but quite well defined, with 

 numerous black gaps running into it, and in general con- 

 forming with the peculiarities of the Milky Way in that 

 region," showing it to be actually mixed up with the Galactic 

 stars, and resembling in this respect a large nebula which 

 Mr. Barnard has photographed in Cepheus. He draws 

 attention to a sinuous vacant strip in this region of the 

 Milky Way running from 1-5 Monocerotis, at first to the 

 west for some three degrees, and then northerly nearly to 

 y Geminorum. 



The photograph of which Mr. Barnard has kindly sent 

 a copy for reproduction in the pages of Knowledge, was 

 taken on February 1st, 1894, with three hours' exposure. 

 The diameter of the great nebula is roughly three degrees. 

 Mr. Barnard describes its character as follows : " It clusters 

 densely about the groups of stars, and then spreads out in 

 a weak, diftuse light, with rifts in it, and irregularly ter- 

 minated along the edges of a vast vacancy in the Milky 

 Way. The condensation, which is very strong, is not at 

 15 Monocerotis, but twelve minutes south, preceding that 

 star, where it becomes a compact mass, with numerous wisps 

 and holes in it. The whole group of three or four bright 

 stars are involved in this denser wispy light, but 15 

 Monocerotis itself does not seem to be particularly con- 

 nected with the nebulosity further than to be apparently 

 in it ; that is, there are no indications of condensation 

 about this the brightest star of the group. This remarkable 

 nebula — the denser part of it — is worthy of study with a 

 more powerful photographic telescope." 



It will be noted that Mr. Barnard drew special attention 

 to the suitability of the brighter portions of this remarkable 

 object for the more detailed study which mii,'ht bo efiected 

 by a more powerful instrument, and the readers of Know- 

 ledge have been put in a position to make such a fuller 

 examination by the courtesy of Dr. Roberts, who supplied 

 the reproduction of his fine photograph taken on February 

 18th, 1895, which appeared in the February Number of 

 the current volume. The scale of Dr. Roberts' photograph, 



• Astronomy and Astrophysics, March, 1894, p. 178. 



taken with the 20-inch reflector, is almost exactly six 

 times that of Mr. Barnard's. A rectangle eighty-five 

 millimetres in height, by eight-one in breadth, with its 

 lower edge thirty-two millimetres from the lower edge of 

 the accompanying photograph, and its west side forty-one 

 millimetres from the corrresponding side of the photograph, 

 would inclose precisely the same portion of the sky as is 

 shown on Dr. Roberts' plate. 



The two photographs are not, however, for comparison, 

 in the ordinary sense of the word. They were taken with 

 widely difierent instruments, for widely datferent purposes ; 

 and each is, in its own special line, of the highest beauty 

 and value. The Willard lens brings up the faint widely- 

 diffused nebulosity which is so striking a feature of the 

 region, and gives what one may call a bird's-eye view of 

 the general stellar distribution ; whilst in the delicacy and 

 fineness of the details of the nebula which it reveals, in the 

 number of stars shown, and in the neatness and precision 

 of their images, the photograph with the reflector is un- 

 approachable. The two photographs are, therefore, in no 

 sense antagonistic ; rather, they are the necessary comple- 

 ments the one of the other. 



Beside the great nebulosity around 15 Monocerotis, the 

 photograph shows another of considerable extent dis- 

 covered by Mr. Barnard whilst sweeping over this region 

 soon after the opening of the Lick Observatory in 1888. 

 It is shown very markedly on the plate as an irregular 

 elliptical mass about thirty-six minutes in diameter. The 

 position of the nebula is given by Mr. Barnard as 

 a=6h. 23m. 27s. J= + 10^ 7' for 1860. 



The entire photograph covers the region between R.A. 

 6h. 17m., and R.A. 6h. 45m. ; declination between 5° 21' 

 and 13" 57' north. 



Noticts of ISoolts. 



A Naturalist in Mid-Africa, beina an Account of a 

 Journey to the Mountains of the Moon and Tangamjiica. 

 By G. F. Scott Elliot, M.A., F.L.S., P.R.G.S. (Innes.) 



Eii))horbiiis ol tlh- AIIhti l.awai-.i I'Imiii-*. i'lviu " A .Natiiniii.'-i 

 in Mid-Africa." 



Illustrated. 163. Mr. Scott Elliot's object in under- 

 taking the expedition of which this book is a description 

 was entirely a botanical one. Unlike most books of travel, 



