Maj' 2, 1878] 



NATURE 



efiforis made in 1874. We should be inclined to hope, 

 however^ that the results of Mr. Gill's expedition to 

 Ascenii-.n, with Lord Lindsay's heliometer, for observa- 

 tions of the recent close opposition of the planet IVIars, 

 may very materially facilitate a decision upon this point. 

 If, as many practical astronomers have anticipated, an 

 equally reliable determination of the sun' s distance can 

 be obtained from measuring heliometrically the diurnal 

 parallax of Mars at those oppositions when he approaches 

 nearest to the earth, as from the observation by the 

 combined exertion of civilised nations, of a transit of 

 Venus, then it maybe reasonably expected that a method 

 admitting of comparatively such frequent repetition, and 

 involving also so small an outlay, not only as to cost, but 

 labour of preparation, must be preferred by astronomers 

 generally, thus facilitating a proper conclusion with 

 respect to expensive preparation for observing the transit 

 in 1882. 



After briefly describing other methods of approximat- 

 ing to the amount of solar parallax which have been 

 applied, including M. Cornu's determination from mea- 

 surement of the velocity of light and Leverrier's results 

 from the planetary theories, and the method first 

 suggested by Prof. Galle, by measuring the parallax of 

 the small planets, a method which, in a modified form, 

 was applied with so much success on the occasion of the 

 near opposition of Juno in 1874, Prof. Newcomb sums 

 up results by stating that " from the general accordance 

 of the various methods described, it would appear that 

 the solar parallax must lie between pretty narrow limits^ 

 probably between 8""82 and 8"*86, and that the distance 

 of the sun in miles- probably lies between the limits 

 92,200,000 and 92,700,000." This, however, it must be 

 observed, appears to have been written before any of the 

 results of the transit [[of Venus were published, as the 

 author expresses the hope that after their discussion the 

 uncertainty may be brought within yet narrower limits. 



The chapter concludes with an outline of the various 

 investigations of stellar parallax, the most trustworthy 

 values being collected in a tabular form at the end of the 

 volume, and the same has been done as regards succes- 

 sive determinations of the mean parallax of the sun. 

 By thus avoiding the introduction of any considerable 

 amount of numerical detail into the text, the volume is 

 rendered much more readable ; indeed, in this respect 

 we may remark, once for all, that the author's arrange- 

 ment leaves nothing to be desired. 



The second part of the work further includes chapters 

 on the motion of light as measured by celestial observa- 

 tions, and experimentally on the methods devised or 

 practised by MM. Foucault, Fizeau, and Cornu ; the 

 application of the revolving wheel is explained. A brief 

 outline of the principles of spectral analysis as applied 

 to the heavenly bodies is presented in conclusion. With 

 reference to doubts which have found expression at 

 times as to the degree of certainty attaching to some of 

 the inferences drawn from spectroscopic observation, and 

 remarking that the dark and bright lines in the spectrum 

 are "the letters of the open book which we are to inter- 

 pret so as to learn what they tell us of the body from 

 which the light came, or the vapours through which it 

 passed," the author introduces the question. How do we 

 know but that the lines we observe may be produced by 



other substances besides those which we find to produce 

 them in our laboratories ? May not the same lines be pro- 

 duced by different substances ? The answer to this question 

 can only be founded upon an appeal to probabilities. "The 

 evidence in this case is much the same as that by which, 

 recognising the picture of a friend, we conclude that it 

 is not the picture of any one else. For anything we can 

 prove to the contrary, another person might have exactly 

 the same features, and might, therefore, make the very 

 same picture. But, as a matter of fact, we know that 

 practically no two men whom we have ever seen do look 

 e.xactly alike, and it is extremely improbable that they 

 ever would look so. The case is the same in spectrum 

 analysis. Among the great number of substances which 

 have been examined with the spectroscope, no two give 

 the same lines. It^is^ therefore^ extremely improbable 

 that a given system of bright lines could be produced by 

 more than one substance." Nevertheless, it is remarked 

 that the evidence of the spectroscope is not necessarily 

 conclusive in all cases. In the case of a single line only 

 of a substance being found in the spectrum of a star or 

 nebula, it would hardly be safe to infer from this alone 

 that the line was really produced by the known substance, 

 In such doubtful instances collateral evidence must be 

 allowed its weight, and conclusions must be drawn with 

 care and discrimination, in accordance with the proba- 

 bilities of each special case. 



The third part of the work is devoted to a description 

 of the sun and planets, the comets and meteors. The 

 author has had the advantage of outlines of the views of 

 several distinguished students of the physical constitution 

 of the sun, which he presents in their own words. These 

 include notes by Father Secchi, M. Faye, and Prof. 

 Young. On the subject of intra-mercurial planets he 

 remarks upon the fact of suspicious objects thus far 

 having been seen only by amateur observers, and escap- 

 ing the skilled astronomers who have occupied themselves 

 in watching the sun's disc, and appears to consider that 

 this circumstance places their real existence beyond 

 moral probability. He favours the idea that if the 

 motion of the perihelion of Mercury be due to the action 

 of a group of planets, they are each so small as 

 to be invisible in transits across the sun, and during 

 total eclipses, and yet being so large in the aggregate, 

 their number must be counted by thousands, and if seen 

 at all they would appear only as a cloud-like mass. The 

 zodiacal light offers this aspect, and the question arises 

 whether the matter which reflects this light can be that 

 which affects the motion of Mercury, and Prof. Newcomb 

 is rather in favour of this explanation, though, he adds, 

 "a great deal of research — more, in fact, than is likely 

 to be applied to the subject during the present generation 

 — will be required before the question can be settled." 



The fourth part treats of the stellar universe, the 

 second chapter on the structure of the heavens meriting 

 especial attention. Space will not allow of our- entering 

 in detail upon the contents of this last portion of the 

 volume ; it concludes with an expression of the author' s 

 ideas relative to the much-discussed question of the 

 Plurality of Worlds. This last part will probably possess 

 greater interest for many than the rest of the work, 

 though necessarily entering upon subjects not yet re- 

 moved from the region of speculation. Prof. Newcomb, 



