3 86 



NA 7 URE 



\_Fcb. 26, 1885 



It appears from this, as from a former example, that it 

 is necessary to take the same word in two perfectly dif- 

 ferent meanings according as it is met with in the first 

 (or ordinary dynamical) part of the book, and in the later 

 (or thermodynamical) part. Such at least is the case 

 with the two specially important terms. Pressure and 

 Efficiency. 



It is perhaps hypercritical to call attention to peculiari- 

 ties of expression which, however they may puzzle him, 

 can scarcely mislead the student. Else we might ask 

 why (p. S) a point is " animated hy any number of veloci- 

 ties," or " subjected to any number of simultaneous veloci- 

 ties," or why "additional velocity" is said in contrast 

 (p. 1 2) to be " received." 



We have marked at least a score of places, in addition 

 to those already noticed, in which the same or similar 

 confusion occurs : — and yet we have read in all only about 

 a fourth of the book here and there, having glanced over 

 the rest much more hastily. But it is enough to have 

 said, while illustrating our remarks by simple instances, 

 that this is certainly not a book for beginners, nor for 

 any one whose hold of the exact meaning of scientific 

 terms is precarious : — though it may be consulted without 

 danger (scarcely, we should think, with actual pleasure) 

 by a student who, already soundly educated in the prin- 

 ■iples of Dynamics, desires to get a rapid and condensed 

 resume of their development by mathematical methods. 



The principle of dual authorship rarely works well in 

 practice. One of the authors of this book invariably 

 speaks of Centre of mass (or of inertia) of a body, the 

 other as invariably of Centre of gravity. And their 

 responsibility has been so thoroughly divided, that neither 

 of these terms is defined, so far as we can find (even with 

 the help of the Index), anywhere in the volume. Again, 

 one of the authors seems to have been always on the 

 look-out to put in a little bit of Kinematics wherever he- 

 had a chance. And surely a third must have been at 

 work, whose function was to stick in some sections on 

 the Rotation of a Rigid Body (p. 92) between the sections 

 on Circular Orbits and those on the Simple Pendulum. 



The extraordinary Olla podrida of Schell is one of the 

 authorities mentioned in the Preface as having been 

 largely borrowed from. The book would certainly have 

 been very much better had that work been let alone ; 

 though no work more richly deserves to be plundered in 

 its turn than does that of Schell, who simply adopts 

 (and too frequently distorts) whatever pleases him. 



OUR BOOK SHELF 

 Les Organismes probUtnatiques des Anciennes Mers. By 



the Marquis de Saporta. (Paris : Masson, 1SS4.) 

 The views expressed in Saporta and Marion's " Evolution 

 des Cryptogames " (reviewed at length in NATURE, vol. 

 xxiv. pp. 73, 558) as to the origin of certain markings 

 commonly met with in pateozoic rocks, has led to a long 

 discussion in which many have taken part, the chief 

 champions on either side being Dr. Nathorst, the dis- 

 tinguished Swedish botanist, and the Marquis de Saporta. 

 Dr. Nathorst maintains that they are tracks left by moving 

 or burrowing animals or other inorganic markings, whil 

 Saporta holds to his original opinion that very many of 

 them are casts of primaeval alga, of kinds now extinct. 

 Nearly all of these markings are in bas-relief on the under 

 surfaces of slabs as if they were moulds of prints or im- 



pressions traced in the ancient muds, thus at first sight 

 greatly favouring Nathorst's view of their origin. Saporta 

 demonstrates on the other hand that this is a by no means 

 uncommon mode of fossilisation among undoubted plants, 

 and when we reflect on the composition of algae, we shall 

 see that scarcely any other mode of fossilisation among 

 them is possible. A leathery olive green sea-weed lying 

 on an oozy mud would cause an indentation, and if sub- 

 sequently covered up, would keep the old surface from 

 contact with the fresh mud, until it might, under favour- 

 able conditions, have become sufficiently hardened to 

 retain the impression. The sea-weed, as most olive 

 weeds do now, if left in water or fresh mud, would eventu- 

 ally completely dissolve away, leaving no perceptible 

 organic trace of its presence. The cavity thus left would 

 be filled in at last by the overlying mud, and only a 

 cleavage plane would remain, following the contour of the 

 under side of the weed, and marking its former presence. 

 Sometimes, though rarely, the sea-weed might not decay 

 until a cleavage plane had been established around its 

 entire circumference, without leaving the smallest trace 

 of its internal structure, as we often find is the case with 

 far more resisting cryptogamic stems in the older rocks. 

 This Saporta finds is the case with the Bilobites, one of 

 the most vexed of all the " Organismes proble'matiques," 

 and he relies with good reason upon their occasional 

 occurrence in this condition and on their reticulated 

 structure to support his contention that they cannot be 

 mere worm tracks or burrows, and that in point of fact 

 they can be naught but the impressions of primordial 

 algae. J. S. G. 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of rejected manuscripts. 

 No notice is taken of anonymous communications . 



[ The Editor urgently requests correspondents to keep their letters 

 ■as short as possible. The pressure on his space is so great 

 that it is impossible otherwise to insure the appearance even 

 of communications containing interesting and novel facts, ] 



Civilisation and Eyesight 

 My attention has only recently been called to a communica- 

 tion horn Lord Rayleigh, which appears in Nature for the 

 1 2th inst. (p. 340), and on which I crave permission to make a 

 few observations. Lord Rayleigh questions whether the eyes 

 of savages, " merely as optical instruments," are greatly supe- 

 rior to our own ; and suggests that any superiority which savages 

 possess may depend upon "attention and practice in the inter- 

 pretation of minute indications." He explains that "the re- 

 solving power of an optical instrument is limited by its aperture," 

 and then proceeds as follows : — 



" With a given aperture no perfection of execution will carry 

 the power to resolve double stars, or stripes alternately dark and 

 bright, beyond a certain point, calculable by the laws of optics 

 from the wave-length of light. With sufficient approximation 

 we may say that a double star cannot be fairly resolved unless 

 its components subtend an angle exceeding that subtended by 

 the wave-length of light at a distance equal to the aperture. If 

 we take the aperture of the eye as one-fifth of an inch, and the 

 wave-length of light as 1 -40,000th of an inch, this angle is found 

 in be ah ml two minutes ; and we are forced to the conclusion 

 that there is no room for the eye of the savage to be much 

 superior in resolving power to these of civilised physicists, whose 

 powers approach at no great distance the theoretical limit as 

 determined by the aperture." 



I understand this to mean 'that optical conditions limit the 

 resolving power of the eye to objects which subtend a visual 

 angle of about two minutes, and that civilised physicists approach 

 this theoretical limit at no great distance. 



Willi great submission to the high authority of Lord Rayleigh, 

 I venture to question whether we have any data from which to 

 draw conclusions with regard to the possible optical powers of 

 the eyes of the human race. We should probably fall into 

 grave error if we were to argue from the reduced eye of Listing, 



