386 



NATURE 



{Feb. 2 1, 1889 



naively says, " He was sent back to the depot, and made 

 no further complaint of weak sight." The author is not 

 complimentary to his confreres when he says that many 

 so-called malingerers are ih reality found to be suffering 

 from eye disease which their surgeons have been unable 

 to detect. 



The manual is profusely illustrated. Many of the 

 illustrations are excellent, but some are of a fearful 

 and wonderful nature. The frontispiece, representing 

 a section of the anterior hemisphere of the globe of the 

 eye, is, in its way, a masterpiece : the knob-like protuber- 

 ance on the iris, the wormy ciliary processes, and the 

 aggressively patent posterior aqueous chamber (which is 

 now known to be non-existent as a space) complete a 

 picture which belongs rather to the domain of comedy 

 than to the province of reality. 



GENERAL ASTRONOMY. 



General Astronomy. By Prof C. A. Young, Ph.D., LL.D. 

 (Boston, U.S.A., and London : Ginn and Co., 1888.) 



PROF, YOUNG is so well known in this country 

 through his researches in solar physics, that he needs 

 no introduction to our readers. The title of his latest work 

 is very comprehensive, but we may at once say that it is not 

 more so than the book itself Every branch of the subject 

 is touched upon more or less, although no particular 

 branch receives very extended treatment. It is essentially 

 a book for students, " and is meant to supply that amount 

 of information upon the subject which may fairly be 

 expected of every liberally educated person." For an 

 intelligent reading of the book, only the most elementary 

 knowledge of mathematics is necessary, but, as pointed 

 out in the preface, the mental discipline and maturity 

 which usually attend the later years of College life are 

 assumed. The student is warned at the outset that 

 astronomy, notwithstanding its important practical ap- 

 plications, is in the main a subject of intellectual pleasure 

 rather than an economic one. 



The general arrangement of subjects is as good as it 

 well can be. Definitions and general considerations come 

 first, then the various instruments are described, and these 

 are followed by the corrections to be applied to instru- 

 mental measurements. Succeeding chapters deal with the 

 various phenomena and problems presented by the earth, 

 moon, sun, planets, comets, meteors, nebulas, and stars. 

 It is not necessary that we should refer in detail to those 

 parts of the book dealing with the " old astronomy.'' 

 Still, it may be mentioned tkat the treatment of Kepler's 

 laws and the theory of lunar perturbations is especially 

 good. We may also recommend Art. 253 to the notice 

 of landscape painters who may be in doubt as to the 

 representation of the moon with scientific accuracy. It 

 would be hard to find a better account of this matter than 

 that given by Prof. Young, and it has the great advantage 

 of brevity. 



It is in the various departments of astronomical physics 

 that most debatable points arise, and any criticism is 

 naturally directed there. With regard to sun-spots, Secchi's 

 theory, with slight modifications, as previously published 

 in Prof Young's well-known book on the sun, is regarded 



as the most probable one. The theory that they are due 

 to falls of cooled materials, however, scarcely receives 

 justice. The objection raised is that it is not in accord- 

 ance with the distribution of spots in latitude, but Mr. 

 Lockyer has fully explained (" Chemistry of the Sun ") 

 how these spot zones may be accounted for by this 

 theory. Obviously, if spots were produced by the fall of 

 meteorites into the sun from any direction whatsoever, 

 and there were no system of regulation, they would be 

 formed all over the surface, and without espect to period. 

 But just such a regulator as is required is to be found in 

 the solar surroundings, and in the system of solar cur- 

 rents, of the existence of which evidence is constantly 

 accumulating. Moreover, by far the greater part of spot- 

 producing material is probably " home-made," consisting 

 of the cool condensed products of the vapours which 

 have been driven out from below. In the face of these 

 explanations, it is difficult to understand why Prof Young 

 should have dismissed the " downrush " hypothesis with 

 so few words. 



Some people find it difficult to believe that a light body 

 like a comet can partake of motion in an orbit just as 

 well as a planet, but these doubters are reminded (p. 407) 

 that a feather falls as freely as a stone in a vacuum, and 

 that this condition holds for space. 



The theory that the luminosity of comets is due to 

 collisions between the meteorites of which they are 

 composed appears to find little favour with Prof. Young. 

 He remarks (p. 418) that, " although the absolute velocity 

 of the comet is extremely great, the r^/a/zV^ velocities of 

 its constituent masses, with reference to each other, must 

 be very slight — far too small apparently to account for 

 any considerable rise in temperature or evolution of light 

 in that way." It must not be forgotten, however, that 

 the meteorites must have some relative velocities, owing 

 to the differential attraction of the sun upon the swarm, 

 and that the disturbance thus set up would be increased 

 as the distance from the sun diminished. The number 

 of collisions would therefore vary exactly as the brightness 

 of comets is known to vary, and this argues strongly in 

 favour of the collision theory of luminosity. Prof. Young 

 attempts to get over the difficulty by suggesting (p. 418) 

 that a gas in the mass may become sensibly luminous at 

 a much Tower temperature than is generally supposed, 

 but this, we fear, is scarcely reconcilable with the kinetic 

 theory of gases. 



There is an excellent short account of the zodiacal 

 light on p. 347, but no reference is made to its connection 

 with the aurora, as regards its spectroscopic phenomena 

 and its periodicity. This relation furnishes further im- 

 portant evidence of its meteoritic nature. 



Prof. Young has evidently very, little sympathy with 

 Mr. Lockyer's new meteoric hypothesis. Only scattered 

 references to it are made, and very little criticism is 

 vouchsafed. The little criticism that there is seems to 

 be based on an imperfect acquaintance with the papers 

 already published. Thus, the collision theory of variable 

 stars is objected to (p. 484) on the ground that it is not 

 consistent with the irregularities in the periods, but Prof. 

 Young is evidently under the impression that this theory 

 limits itself to the case of a single cometic swarm re- 

 volving in an orbit round a central swarm, whereas this 

 case was put forward as the simplest possible. 



