188 



KNOWLEDGE 



[OOTOBEB 1, 1891. 



been protruded from below. It is not definitely known 

 whetber tbe diamonds were already formed in tbe rock 

 before its eruption, or whether they had been produced 

 by alteration of the materials contained in the rock 

 displaced by the eruption. It is worthy of notice, however, 

 that a blacl; shale forms one of the surrounding rocks, and 

 pieces of this shale have been found baked and otherwise 

 altered in the " blue rock." The suggestion has been 

 thrown out that the diamonds were formed by the altera- 

 tion of the carbonaceous matter of the shale, under the 

 influence of a moderately high temperature and great 

 pressure. Such indications of origin are useful as 

 affording suggestions to the experimentalist, to whom, 

 however, in spite of previous failures, we must still look to 

 tell us definitely how the diamond is formed. 



XoTE. — I am indebted for most of tlie information on the 

 methods of workins; in the mines to mv friend Mr. A. R. Sawyer, 

 A.R.S.M., formerly one of H. M. Insjicctors of Mines, now resident 

 ill Soutli Africa. 



ON THE DISTANCE AND STRUCTURE OF THE 

 MILKY WAY IN CYGNUS. 



By A. C. Eanyakd. 



IA:\I indebted to Dr. Max Wolf, of Heidelberg, for 

 the beautiful photographs of the Milky Way which 

 illustrate this paper. The large plate, which repre- 

 S(?nts the region about a Cygni, is enlarged from a 

 photograph taken with an exposm-e of thirteen 

 hours and five minutes, given on two successive days — 

 viz. : 1891, September 9th, 9h. Om. to 15h. 30m., and 

 September 10th, 9h. Om. to 15h. 35m. — with a camera of 

 134 miUimetres (that is, about 5j inches) aperture, and a 

 focal length of 770 millimetres (that is, about 30^ inches). 

 The camera, therefore, is of about the same focal length 

 as the cameras used by Mr. Eussell in Sydney, and 

 Professor Barnard at the Lick Observatory, though the 

 pencils of light falling on Dr. Max "\\'olf's plate were only 

 about three-quarters as intense as those falling on the 

 plates in the cameras of the Sydney and Lick Observa- 

 tories ; the aperture of Dr. Max Wolfs camera being 

 5'277 inches, as compared with a full six inches of aperture 

 at the Lick and Sydney Observatories. During the long 

 exposures Dr. Max Wolf was relieved by Messrs. Staus and 

 Kosenplaenter in keeping the camera continuously directed 

 to the region photographed. It will be seen that in all 

 three plates the motion of the stars has been very satis- 

 factorily followed. 



The region shown in the large photograph is specially 

 interesting for many reasons. It is a region rich in red 

 stars, and also rich in the small class of stars which 

 show bright lines in their spectra, and are known as 

 Wolf-Eayet stars. It corresponds to the northern end 

 of the great rift which di^-ides the Milky Way into two 

 branches throughout half of its course roimd the heavens, 

 and it is a part of the ]\Iilky Way crossed by the 

 zone of large stars referred to in the May and July 

 numbers, as probably being at about the same distance 

 from us as the Milky Way stream of nebulous light and 

 small stars. Dr. Gould, speaking of this zone of large 

 stars, says : '■'■ " Few celestial phenomena are more pal- 

 pable than the existence of a stream or belt of bright stars 

 traceable with tolerable distinctness through the entire 

 circuit of the heavens, and forming a great circle as well 

 defined as that of the Galaxy itself, which it crosses at an 

 angle of about 20^ in Crux and Cassiopeia. Traversing in 

 the southern hemisphere Orion, Canis Major, Argo, the 



* T!,e Amerimi? Journnl of Scieiire, vol. viii., )). 333. 



Centaur, Lupus and Scorpio, it pursues its way in the 

 northern through Taurus, Perseus, Cassiopeia, Cepheus, 

 Cygnus, and Ijyra, its line being less obviously continued 

 by the stars of Hercules and Ophiuchus." 



It will be noticed that the two large stars in the picture (a 

 and y Cygni) are surrounded by a nebulosity very similar in 

 appearance to other masses of nebulosity evidently asso- 

 ciated with small stars. Thus the lower portion of the bright 

 cloud to the right hand of the picture is composed of a 

 milky nebulosity evidently associated with the stars which 

 lie along its curving border, while the upper part of the 

 same cloud-mass is more granular, and seems to consist of 

 a multitude of small stars on a background of fainter nebu- 

 losity. The association of these large stars with nebu- 

 losity which certainly, in the case of y Cygni, seems to 

 extend into and form a part of a larger nebulous mass 

 evidently associated with the small stars of the Milky Way, 

 is another important link in the chain of evidence tending 

 to show that the zone of large stars above referred to is at 

 the same distance from us as the small stars of the Milky 

 Way. 



The star a Cygni is of the 1-5 magnitude according to the 

 Harvard Photometric Catalogue, and we may safely assume 

 that the smallest stars shown on the photograph which 

 seem to be associated with the cloud-masses are not 

 brighter than the 18th magnitude of the photometric scale. 

 A difference of 16} magnitudes between two stars at the 

 same distance from us, means that the brighter star must 

 be giving about four million times as much light as the 

 smaller one ; that is, if the larger star had a photosphere 

 as bright as the photosphere of our sun, and a diameter 

 ten times as great, so that it gave a hundred times as 

 much light as our sun, the small stars would, if they had 

 photospheres as bright as the photosphere of our sun, have 

 a diameter only a little greater than half the diameter of 

 the earth. f 



Thus, imless we suppose the diameter of a Cygni, and 

 the other large stars associated with the Milky Way, to be 

 enormously great, or unless we suppose them to be 

 intensely bright compared with our sun, the smaller stars 

 associated with them in the Milky Way must be very 

 minute compared with our sun. The actual size of these 

 suns is a matter of considerable interest to us, for if we 

 knew the size and brightness of any of these stars, or if we 

 could make any approximate estimate as to their light- 

 giving power, we should have a means of determining the 

 distance of the Milky Way. Thus, if we knew that 

 a Cj'gni did not give a hundred times as much light as 

 our sun, we should know that this region of the Milky Way 

 could not be ten times as distant as a. Cmtnuri, our nearest 

 neighbour amongst the stars, for our sun would probably 

 appear less than a star of the 1'5 magnitude if it was 

 removed to a distance from us equal to the distance of 

 a C<-ntaurL 



Unless, therefore, we are prepared to believe that a Cygni, 

 and all the other large stars associated with the Milky 

 Way, are on altogether a different scale from our sun, as 

 well as from a Cfnt(n(ri and the other stars whose magni- 

 tude or brightness we have been able roughly to estimate, 

 we must assume that the Milky Way is not more than ten 

 times as distant from us as a, Centauri. 



Figs. 1 and 2 have been made by a photographic pro- 

 cess so as to show the chief stars in Cygni, recognizable 

 in the two photographs on our second plate. With the 



t If, on the other hand, the IStli niagnitnde stars associated with 

 a Cygni ai-e as large and as bright as our sun, a C'vgui itself would 

 need to have a diameter as great as the diameter of the orbit of Saturn, 

 if its photosphere was only as bright as the photosphere of our sun. 



