36: 



NA TURE 



[August 17, 1893 



without regard to any preconceived ideas or theories. 

 The distribution of nebulas is then considered, and a care- 

 ful analysis made of the many wonderful structures 

 shown in Dr. Robert's photograph of the great nebula 

 in Orion. The great similarity of these forms to those 

 traceable in the Solar Coronals clearly demonstrated, and 

 it is suggested that just as the coronal forms probably have 

 their origin in enormous streams of gaseous matter 

 ejected into a resisting medium, so these similar 

 structures may be due to a similar cause. Mr. Ranyard 

 then shows how these forms are reproduced in the 

 arrangement of clouds of stars in the Milky Way, as 

 shown by the marvellous photographs taken by Prof. E. 

 E. Barnard of the Lick Observatory, e.«ellent reproduc- 

 tions of which illustrate this portion of the book. A 

 detailed examination of Prof. Barnard's plates seems to 

 Mr. Ranyard to indicate the existence of dark-absorbing 

 matter, "either like cold gas or fog of opaque particles in 

 space, cutting out or dimming down the light of the 

 region beyond." These dark patches assume the curved 

 forms and tree-like structure already referred to, and thus 

 seem to further confirm the idea of a resisting medium, 

 which, as Mr. Ranyard is careful to point out, need not 

 necessarily be a gas ; dust m.oving in space, or meteors, 

 or large masses would equally offer resistance. 



By deliberately overprinting photographs of the Milky 

 Way, long chains of stars and curving dark lanes have been 

 brought into great prominence, and have materially 

 assisted in the investigation. There is certainly much to 

 recommend this startling suggestion of dark absorbing 

 matter in space, and the wonderful details of Prof. 

 Barnard's photographs, and the similarity to coronal and 

 nebular forms, can scarcely be explained as due to acci- 

 dental groupings of stars and dark spaces in the Milky 

 Way. Although most authorities, including Prof. Barnard 

 himself, prefer to suspend their judgment in the matter 

 until still more photographic results are available, there 

 can be little doubt that no more satisfactory hypothesis 

 has as yet been advanced. 



The connection between nebula; and bright stars, and 

 the connection of bright stars with faint ones by means 

 of thin wisps of nebulous matter, undoubtedly indicate 

 that dififerences in magnitude of stars are due to dififeren- 

 ces of physical condition and not to distance. As illus- 

 trating the far reaching results of this conclusion Mr. 

 Ranyard says : — 



" If we assume a distance fifteen times as great as the 

 distance of a Centauri, for a part of the Milky Way in 

 which a first magnitude star is found to be associated 

 with stars of the lyi magnitude, we must be prepared to 

 assume a diameter for the large star twenty times as great 

 as the solar diameter, unless its photosphere is brighter 

 than the solar photosphere ; while the smaller stars if 

 their photospheres were as brilliant as the solar photo- 

 sphere, would have diameters equal to about one- 

 hundredth part of the solar diameter— that is, they would 

 not much exceed the earth in magnitude." 



The question of proper motion next receives atten- 

 tion, and this is followed by an account of binary and 

 triple stars, most of the recent work being fully dealt 

 with. In the discussion of stellar spectroscopy we find 

 Secchi's classification of star spectra given to the exclu- 

 sion of all others. Details of other systems might well 

 NO. 1242, VOL. 48] 



have been introduced here, but it is evident that Mr. 

 Ranyard considers the subject one in which very little 

 advance towards a proper classification has been made. 

 He is of opinion (p. 79.,) that the Sirian type are less con- 

 densed and are in an earlier stage than the solar type, 

 and indicates that bright line stars must come somewhere 

 between nebula; and the Sirian type ; but there is nothing 

 to indicate whether he thinks this is the stage of rising 

 or falling temperature, and the whole question is left in 

 a vague and somewhat unsatisfac'tory manner. 



In considering the supposed physical connection be- 

 tween the stars in the great nebula in Orion and the 

 nebula itself, Mr. Ranyard relies on the fact that similar 

 bright lines are found in the spectra of each, and quotes 

 the photographs of Dr. Huggins as proving that these 

 stars are physically bound up with the gaseous matter 

 of the nebula. He himself seems inclined to the opinion 

 that they are not condensations of the nebula;, but are 

 the centres from which the matter now forming the 

 nebula was ejected ; but whether condensations of the 

 nebula or points of origin the spectra are supposed to be 

 similar. It is important to remember, however, tliat it 

 is nearly impossible to get a photograph of the spectrum 

 of a star involved in a nebula without also obtaining a 

 superposed spectrum of the nebula itself. Every tremor 

 of the telescope sufficient to carry the star image off the 

 slit will allow the nebula to imprint its spectrum on the 

 plate, and if the star is allowed to trail along the slit it is 

 clear that the nebula gets more exposure than the star at 

 any particular point in the resulting photograph. There 

 is at present no absolute photographic proof that iho stars 

 in the nebula in Orion contain the nebular lines as bright 

 lines in their spectra, and consequently conclusions based 

 on this assumption are untrustworthy. 



Mr. Ranyard classes the nebuire which give fain: con- 

 tinuous spectra and do not show the characteristic green 

 line, as white nebuLx, and places in this cla=s the 

 Andromeda nebula, the spiral in Canes Venatici, and 

 the nebulous background of the Milky Way, but inakes 

 no suggestion as to the stage of development of these 

 bodies. 



On the question of "What is a nebula ?" it is extremely 

 difficult to understand the exact position assumed by Mr. 

 Ranyard. He evidently considers nebulae as containing 

 solid or liquid matter and as increasing in temperature, 

 but "the very great transparency" renders it probable 

 that they either contain very littl; solid or liquid matter, 

 or that the solid or liquid matter is aggregated into dis- 

 crete masses with an average diameter of more than an 

 inch ; if the density of a nebula, leaving out of account 

 its gaseous constituents, is as much as one one-thousand 

 millionth of the density of atmospheric air at the sea level 

 These conclusions are practically an acceptance of the 

 main idea of Lockyer's meteoritic hypothesis so far as it 

 deals with nebulas, although Mr. Ranyard rejects the 

 spectroscopic evidence bearing on the point. The specij' 

 lations as to the probable density of the Great Orion., 

 nebula, although extremely interesting, are vitiated b^ 

 the fact that it is impossible to estimate the gravitational' 

 effect of the dark matter in interstellar space. 



The book is well and copiously illustrated throughout, 

 the plates and the photographic reproductions being of 

 a very high-class character. Mr. Proctor's portion is 



