362 



NATURE 



[August 16, 1900 



In Chapter v. a good elementary account of the laws of 

 capillarity is given. On p. 444 Quincke's falling drop 

 method of measuring surface tensions is described, the 

 weight of the drop being stated to be equal to the pro- 

 duct of the surface tension and the circumference of the 

 line of contact. Lord Rayleigh has shown that this is 

 not correct even if the liquid motion in the drop at the 

 moment of separation be neglected ; the excess of pres- 

 sure in the drop corresponding to the curvature of the 

 surface (supposed cylindrical near the plane of contact) 

 has the effect of diminishing the size of the drop to one- 

 half the value stated, and this result agrees more closely 

 with experiment. 



The second part of the volume, on sound, resembles in 

 its general mode of treatment most other elementary 

 text-books on the subject. The general nature of wave- 

 motion is made quite clear by numerous diagrams of 



wave-curves and wave-machines^ The deduction given 



on p. 638 of the expression V^/D for the velocity of 

 propagation of sound-waves is not satisfactory, since it 

 involves the tacit assumption that the whole energy is 

 half potential and half ki netic. 



In connection with the experimental measurement of 

 the velocity of sound in water in tubes, referred to on 

 p. 643, the influence of the walls and Kundt's measure- 

 ments in tubes with walls of different thicknesses should 

 have been mentioned, and in the description of the 

 resonance tube experiment, no method is given for 

 eliminating the end correction. 



The last chapter contains an interesting account of 

 the researches of von Helmholtz and others on the 

 vibrations of violin strings, combination tones, analysis 

 of sounds, and the theory of consonance and dissonance. 



In the second volume (light and heat) the author 

 is assisted by Dr. Lummer, who, we are told in the 

 preface, is chiefly responsible for the part dealing with 

 optical systems and the theory of optical instruments. 

 This part of the work has been largely re- written for the 

 present edition, and brought well into line with the 

 modern views on image-formation founded by Abbe. 



As is the case in doing most things, there are two 

 ways of writing a book on geometrical optics. The first, 

 until recently the usual, method is to begin with very 

 special cases, such as thin lenses, and proceed by degrees 

 to the more general cases of thick lenses and systems 

 of lenses. 



The other, and more modern, method is to begin with 

 the general case of a point-point correspondence between 

 two portions of space, of such a kind that to a pencil 

 of rectilinear rays passing through a point in one region 

 corresponds a pencil of rectilinear rays passing through 

 a point (the image) in the other region ; then to intro- 

 duce the special cases of image-formation by reflecting 

 or refracting surfaces and centred systems, including 

 lenses. The two methods thus proceed on opposite 

 lines. 



The latter method has been perfected by Abbe, and 

 is the one adopted by Czapski and, though necessarily 

 in a more elementary and restricted manner, in the 

 present work. 



After two chapters dealing with the nature of light, 

 photometry, refraction and reflection at plane surfaces, 

 Chapter iii. treats of the formation of images by refraction 



NO ibo;. VOL. 62] 



at a single spherical surface ; then the general case of any 

 number of spherical surfaces separating different media, 

 with their centres in a straight line ; and, finally, two co- 

 axial centred systems, with the special case of a " tele- 

 scopic" system in which the " interval " is zero. The lens 

 is regarded as two centred systems, each consisting of a 

 single spherical surface. 



Chapter xii. is devoted to the theory and use of " stops," 

 the calculation of magnifying power and brightness of 

 images in centred systems, and, finally, the laws of 

 formation of images of illuminated objects, as in the 

 ordinary use of the microscope. Here purely geometrical 

 methods break down, and diffraction spectra play an all- 

 important part. A highly interesting account follows o 

 Abbe's theory of microscope images and its remarkable 

 verification by the use of the diffraction plate, in which is 

 shown how the similarity of image to object, as well as 

 the resolving power of the instrument, depends upon the 

 number of diffraction spectra whose rays enter the objec- 

 tive and take part in the final image-formation. How these 

 principles are applied in the construction of microscope- 

 objectives is set forth in the chapter on optical instru- 

 ments, which also contains details of many of the latest 

 improvements in optical instruments of all kinds. 



The second part of vol. ii. (on heat) does not differ 

 from the corresponding part of the previous edition so 

 fundamentally as is the case with the part on optics ; but 

 it is brought more nearly up-to-date by many additions,, 

 including the work of Olszewski and Linde on the lique- 

 faction of gases, a chapter on thermochemistry, steam 

 calorimeters, recent determinations of the specific heat of 

 water at various temperatures and of the mechanical 

 equivalent of heat. No reference is made, however, to 

 recent improvements in the choice of a unit of heat. 



Thermodynamics does not receive very much attention,, 

 few applications being mentioned beyond Kelvin's defini- 

 tion of absolute temperature, and a calculation of the 

 change of melting point produced by pressure. Some 

 details are given, however, of the parts and action of 

 steam, air, and gas engines. 



A short chapter on meteorology, dealing with climatic 

 conditions and their changes, brings this volume to a 

 close. 



As to the third volume, it suffers, as was remarked 

 before, in comparison with the other volumes from having 

 been written several years ago. Still, it contains a large 

 mass of useful information about electrical and electro- 

 technical apparatus, much of which is not usually found 

 in text-books on electricity and magnetism. 



It is impossible, in the space at our disposal, to give 

 more than a very rough sketch of a work which extends 

 to close upon 4000 pages, and many excellent qualities of 

 the work must for this reason remain unmentioned. One 

 of the chief features is the large number (nearly 3000) of 

 excellent illustrations, and, chiefly in the section oi> 

 optics, some very beautiful coloured plates. Explana- 

 tions are, as a rule, given very clearly, and often aided 

 by numerical examples worked out. Further, on account 

 of the very large number of experiments and forms of 

 apparatus described, as well as the numerous references 

 to original papers, the work is certain to prove useful, as 

 it no doubt has already done, to students and teachers of 

 physics. 



