38 



KNOWLEDGE. 



[Feurdaky, 1903. 



In treating of the influence of the cover-glass, special emphasis 

 js laid on the effect produced by the variation of thickness of 

 cover when oil-immersion lenses are used. As a matter of fact, 

 these lenses are theoreticallj' least susceptible to this influence. 

 on account of the homogeneity of the medium which forms 

 contact between the front lens and the cover-glass, and herein 

 Ilea one of the great advantages of oil-immersion lenses. 



The description of the polarisation of light is similarly 

 misleading, and the statement is made that the dark field is 

 obtained when the two Xicol's prisms " are almost at right 

 angles." 



Space does not allow of further extracts, but the book 

 generally leads to the conclusion that the wide range of know- 

 ledge which is so essential in those who give directions and 

 advice to others is lacking in the experience of the writer, and 

 that his information on optical subjects has been gleaned from 

 an acquaintance with a verj' limited amount of apparatus and 

 knowledge of its general princijiles. There is here material for 

 a really practical and useful book, but the information it gives 

 should be reliable to justify its existence. 



"Plti!Lic.\T[ons or West Hendox Horsr: Observ.\tory, 

 SoNor.uLANi).'' Ko. II. By T. W. Biekhouse, f.k..\.s. — Five 

 subjects are included in this most valuable work, which, it 

 should be observed, is not a finished treatise on any one of the 

 sections treated, but a mine from which others may dig the ore 

 that they will mould to their own uses. The sections are: — 

 Part II. of The Structure of the Sidereal Universe; Comets 

 Bernard (1 880) and Holmes (1892); The Zodiacal Light; The 

 Aurora Borealis; and Variable and Suspected Variable Stars. 

 In the first section Mr. Backhouse studies the drawings by 

 Easton and Boeddicker, and the photographs by Dr. Isaac 

 Koberts, Prof. Barnard, Prof. Max Wolf, Mrs. Maunder and 

 others published in IvKOWLi.nc.E or elsewhere, and compares the 

 results of various markings or orderings of stars on these with 

 the study of the same regions which he has made visually with 

 a 4]-inch Cooke refractor. His conclusions are most definite in 

 the regions of the Milky Way, whose effect he judges to be 

 built up for the naked eye by aggregations of stars below the 

 10th in magnitude. In other regions of the sky he confirms the 

 existence of many streams of stars and rifts, but can come to no 

 definite conclusion as to whether radial series of streams and 

 rifts do actually exist as such or whether they are simple efl:'ects 

 due to the accidental crossing of two or more series of parallel 

 streams. In this connection we think some light might be 

 shown by a well-marked feature of many of Dr. Roberts' 

 photographs, namely, the spiral arrangement of curved lines of 

 stars (not nebular spirals), which might well give in some cases 

 the effect of radiating spokes of stars. We do not find that 

 Mr. Backhouse refers to or has studied this spiral formation. 



His observations of the place of the Counterglow extend 

 from 1871 to 189."), but he says that, looking at the whole 

 of these observations, it seems impossible to arrive at any 

 conclusion as to the inclination of the Zodiacal Light to the 

 Ecliptic, there being but a very slight preponderance in favour 

 of an ascending node about longitude lit''. The observation of 

 aurorsE have become increasingly difficult with the increase in 

 size and the better lighting of the town. His observations 

 tend to confirm slightly the contention of Dr. Veeder, that when 

 an aurora is seen there is a tendency for the phenomenon to 

 recur when the same part of the sun's disc is again presented to 

 the earth ; but he thinks that little weight can be given to it as 

 this so nearly coincides with a lunation, which itself gives a 

 false ])eriodicity in the observation of aurorie. 



Each section is illustrated by several very fine plates. 



"Ble.men't.\ry Photo-Micrography." By W. Bagshaw. 



(London : Iliffe & Sons, Limited. 1902.) Illustrated, ls.net. 

 — This book, while recognising the advantages derivable from 

 the use of the btst means for the work, indicates in a simple yet 

 direct manner how photo-micrography may be done with simple 

 instruments and contrivances, and the use of common sense. 

 Work in such circumstances necessarily has its limitations, but ] 

 as there is a tendency in the majority of bo;ks on photo- 

 micrography to treat the subject as though every worker had 

 the purse of " Fortunatus," it may induce those whose resources 

 are limited to attempt this fascinating work ; and not to such 

 alone, but to photo-micrographers generally, this little book 

 will be found to contain many useful hints, based on sound, 

 practical knowledge. 



" Practicai. PiioTO-Mrf'ROGRAPiiv." By Andrew Pringle. 

 (Iliffe & Sons.) — This is the third edition of a work which 

 has become a standard on the particular subject with which 

 it deals, and the writer is one who so graphically describes 

 his own practical methods of working as to enable the reader 

 to grasp the directions given and realise the results described. 

 The present issue, which is printed on art paper, and has 

 numerous appropriate illustrations, has been brought up to date 

 in every respect, and the consideration given to the subject of 

 dry plates corrected for colour will be found of special interest. 

 Throughout the book there is no tendency to stint apparatus ; 

 it is practically a disquisition on how to do the best work with 

 the best apparatus, although many suggestions to the worker of 

 modest means are given. Photo-micrographers generally will 

 find this new edition of Mr. Pringle's book a valuable help in 

 their work. 



" 0\ AN ItJVERSiox oi- Ideas as to the Structure op 

 THE U.NIVERSE," By Prof . Osborne Reynolds, f.r.s. Pp.44. 

 Illustrated. (Cambridge: University Press. 1902.) Is. (jd. — 

 Whatever may be the ultimate decision as to the validity of 

 Prof. Rej'nolds's explanation of the nature of the aether and 

 matter, his contribution to the study of this subject must be 

 regarded as of the highest importance. In the little volume 

 under notice, containing the Rede Lecture delivered at Cam- 

 bridge in June last, we have merely the outlines of a bold 

 theory, the details of which will be published later. The 

 experiments described to illustrate the theory are so remarkable 

 that they alone make the book a desirable possession for teachers 

 and students of physics. Briefly, the view put forward by 

 Prof. Reynolds, and held to be sufficient to provide a mechanical 

 explanation of the phenomena and effects of light and gravita- 

 tion, is that the aether or medium existing throughout our 

 universe consists of inconceivably small grains. Matter really 

 represents a deficiency of these grains, causing a strain in the 

 medium and thus accounting for the law of gravitation. The 

 grains have diameters equal to the .seven hundred thousand 

 millionth part of the wave-length of violet light, and their mean 

 path is equal to the four hundred thousand millionth part of 

 the diameter ; their velocity is about one and one-third feet per 

 second. The mean density of this universal medium is stated 

 to be ten thousand times greater than that of water, or four 

 hundred and eighty times greater than that of the densest 

 matter on the earth. The mean pressure is nearly seven 

 hundred and fifty thousand tons on the square inch, being 

 more than three thousand times greater than the strongest 

 material can sustain. From the assumed co-efficient of 

 transverse elasticity of this hypothetical medium the rate of the 

 transverse wave is found to be that of light, while that of 

 longitudinal waves is two and four-tenths greater. It is im- 

 possible here to discuss the evidence upon which Prof. Reynolds 

 bases these conclusions, but we do not hesitate to .say that, both 

 as regards foundation and superstructure the theory represents 

 an inspiring, if only imaginative, view of the con*tii,ation of 

 the physical universe. 



"The Dominio.n of the Air; the Story of AiiiiiAi, 

 Navigatiox." By the Rev. J. M. Bacon. Pp. :j&2. Illustrated. 

 (Cassell.) 6s. — Mr. Bacon has produced a very interesting and 

 valuable book, in which he describes not only the popular 

 aspects of aeronautics from raediceval times to the present day, 

 but also gives attention to the scientific results which have been 

 obtained. It is a great convenience to have in a compact form 

 a trustworthy account of what has been accomplished in recent 

 years in ballooning and other means of ai'rial navigation, and to 

 trace the development of the airships from the imaginative 

 structures of Roger Bacon and Father Lana to dirigible balloons 

 like that of Santos Dumont. Mr. Bacon has himself taken no 

 slight part in what may be termed the renaissance of ballooning, 

 and his own observations and conclusions contribute to make 

 his book a noteworthy one. It is usually stated that Lunardi 

 was the first successful aeronaut on British soil, but Mr. Bicon 

 shows that the credit belongs to a Mr. Tytler, who made an 

 ascent on August 27th, 1784, more than a month before 

 Lunardi's ai'rial voyage from London to Ware. The duration 

 of Lunardi's trip was 2 hours 15 minutes, which is a very good 

 record considering that no free balloon can even now remain 

 aloft for much more than thirty-six hours. There is little 

 prospect of great progress, in ballooning as a means of aerial 

 locomotion. By carrying liquefied gas it might be possible to 



