410 



NATURE 



[August 22, 1907 



Blakesley has given a simple geometrical means of 

 designing a thin lens to comply with these conditions. 

 Of course, spherical aberration is not the only, or even 

 the most important defect of a lens; but the simplicitv 

 of the geometrical construction leads one to wish that 

 expert mathematicians would devote some attention to 

 the subject to see whether graphical methods could not 

 be used in other cases. 



Central and eccentric oblique refractions arc dis- 

 cussed in chapters v. and vi. respectively. Eccentric 

 oblique refraction is answerable for the phenomenon 

 of "coma" or "side flare," which is discussed in 

 great detail in chapters viii. and viiia. It would be 

 impossible, in the short space of an article such as the 

 present, to deal with the author's treatment of this in- 

 teresting subject in detail; it must sufTice to say that 

 it has now for the first time been brought within the 

 reach of any reader possessing ordinary mathematical 

 attainments who will devote the necessary time and 

 attention to the subject. Some of Mr. Taylor's results 

 are similar to those obtain(>d by Von Siedol, but many 

 are novel. The most important advance effected by 

 Mr. Taylor is the investigation of the foci of oblique 

 and eccentric pencils of large aperture. 



The distortion of the image formed by a system of 

 lenses is very fully investigated in chapter ix., where 

 it is shown that Coddington's method is defective in 

 not carrying the spherical aberration of the first lens 

 through to all succeeding lenses, a considerable error 

 being thus introduced. The distortion produced by 

 several combinations of lenses is worked out numeric- 

 ally, and it is shown that, in the case of an eye-piece 

 of a telescope or microscope, an image which is really 

 distorted tnay appear to be undistorted, o-'ing to a 

 peculiarity of the eye. .\chromatism is de.ilt with in 

 chapter x. In reading this, and, indeed, most other 

 chapters of the book, one cannot help being struck 

 by the care with which the author has experimentally 

 tested the results obtained, somejimes finding that an 

 extension of the theory is necessary (see, for example, 

 p. 309). A brief sketch of the errors of the third order 

 is given in chapter xi. 



On closing Mr. Taylor's book, we are left to re- 

 flect on the living interest which he has given to 

 mathematical investigations, essentially of a somewhat 

 clumsy nature. Throughout the book, theory and 

 practice go hand in hand, and we feel that the labour 

 of solving the complicated problems which arise is well 

 worth the while, for something tangible and useful is 

 gained in the end. It would be well if the examining 

 bodies.of the various universities were to attach greater 

 importance to geometrical optics, studied from an 

 essentially practical point of view. -At present the 

 startling discoveries which have been made during 

 recent years in other branches of physics absorb so 

 much attention that many students who sit for ad- 

 vanced examinations in physics are culpably ignorant 

 as to even fundamental properties of lenses. Questions 

 on geometrical optics are rarely set by examiners ; and 

 when they are, they are too often merely mathematical 

 exercises. Since accurate experimenting so often 

 involves the use of lenses and other optical appliances, 

 this state of things is greatly to be regretted. 



Edwin Edser. 

 NO. 1973, '^'OL. 76] 



A THEORY OF THE .ETHER. 



/Ether: .4 Theory of the Nature of Mthcr and of its 

 Place in the Universe. By Dr. Hugh Woods. Pp. 

 xii+ioo. (London: The Electrician Printing 



and Publishing Co., Ltd., n.d.) Price 45. 6d. net. 

 TpmS book is a more elaborated presentation of the 



■»■ views as to the nature of jether set forth by the 

 author in a pamphlet published in iSoS. The zether is 

 " regarded as possessing properties such as might 

 justify its being described as a gaseous fluid, composed 

 of atoms almost indefinitely small as compared with 

 recognised chemical atoms." .'Vgain, " yEther is 

 a fluid whose ultimate particles, or atoms, are 

 so small that they pass into the minute crevices 

 of spaces in the most solid bodies." This view 

 has much in common with some of the older 

 theories of the aether, and is almost identical with that 

 proposed by Mendeleeff in his tract, " An Attempt 

 towards a Chemical Conception of the /Ether " (1902), 

 and which is referred to by the author in support of 

 his views. No attempt is made to overcome the objec- 

 tion first urged by Maxwell to any theory as to the 

 nature of the aether which postulates a discrete struc- 

 ture for it — that all the energy of the universe would 

 have been transferred to it — and the same objection 

 applies even if the aether is regarded as a limiting case 

 of a medium possessing such a structure. 



The theory proposed by the author cannot, there- 

 fore, be accepted as an ultimate theory of the aether. 

 There remains the question whether this idea of the 

 aether affords a satisfactory working model which 

 could be used to give a concrete representation of 

 physical and chemical phenomena, and enable their 

 course to be definitely followed. The theory is applied 

 to a wide range of phenomena, including gravitation, 

 chemical changes and reactions, heat, light, electricity 

 and_ magnetism. Many of the explanations that are 

 claimed as consequences of the particular theory 

 would follow from any theory of the aether that assigns 

 to it the fundamental properties of a moving system. 

 The reasoning is in general vague, and the argument 

 is never pushed far enough to enable a quantitative 

 comparison to be made. A few examples will suffice 

 to .show the character of the reasoning. On p. 3 

 the following argument is given : — 



" The solar system appears to move through space, 

 borne along in an enormous volume of swiftly flowing 

 aether. Now the resistance offered to the free flow 

 of the aether by the partially impervious bodies floating 

 in it is evidently greatest in the line of greatest thick- 

 ness of each bodv, and less as the thickness becomes 

 diminished. .Vccordingly a diffrrence of momentum 

 is thereby caused in the mass of ather, dashing against 

 the body, and there results a current in the aether from 

 places of higher momentum to places where the 

 momentum is lower, with the effect that a whirl, such 

 as occurs in the air under similar circumstances, is 

 produced. These whirls, then, by their continual 

 action, make the bodies more or less spherical, and 

 set them rotating, each on its largest axis, while the 

 whirls, spreading out in ever widening circles, influ- 

 ence the movements of other bodies floating in the 

 same medium." "In this way, the movements and 

 mutual influence of the heavenly bodies may be ex- 

 plained, in a perfectly rational manner, and without 

 imagining any occult power of attraction." 



