628 



NA TURE 



[November i i, 1922 



philosopher afterwards used." Dr. Bell has the same 

 authority for the statement that, when investigating 

 the relationship of dispersion to mean refraction, 

 Newton mixed sugar of lead with the water. Traced 

 to its source, however, this so-called fact appears to be 

 merely a suggestion of Mr. Michell, a friend of Dr. 

 Priestley, offered as an explanation of an otherwise 

 inexplicable experimental result and based on Newton's 

 use of saccharum saturni in other experiments. 



Flint glass good enough for quantitative observation 

 could scarcely have existed in 1666, for about a hundred 

 and forty years elapsed before Guinand resolved the 

 optical glass problem. It was more the absence of 

 suitable material that " delayed the production of the 

 achromatic telescope by some three-quarters of a 

 century " than any action of Newton. Indeed, Sir 

 Isaac Newton should be honoured for his prescience in 

 recognising that in the circumstances the practice of 

 astronomy could best be advanced by the development 

 of the reflector. 



" The Telescope " has been " written for the many 

 observers who use telescopes for study or pleasure and 

 desire more information about their construction and 

 properties," the information hitherto published on the 

 subject being " for the most part scattered through 

 papers in three or four languages and quite inaccessible 

 to the ordinary reader." Within the limits of a single 

 volume the author has collected a great deal of essential 

 information that the general reader will find both 

 useful and interesting. 



Following the historical introduction there are 

 chapters on the modern telescope, optical glass and its 

 working, properties of objectives and mirrors, mount- 

 ings, eyepieces, hand telescopes and binoculars, acces- 

 sories, the testing and care of telescopes, setting up 

 and housing, seeing and magnification, and finally, a 

 brief appendix on work for the telescope. 



The book is not free from mistakes. In the descrip- 

 tion, for example, of the Galilean glass, the field is 

 stated to be approximately measured by the angle 

 subtended at the centre of the objective by the pupil. 

 The description given in " The Telescope " by Herschel, 

 although theoretically incomplete, might have been 

 copied with advantage, as it explains how the field is 

 determined by the diameter of the objective and the 

 possible displacement of the eye. In chapter 7 the 

 so-called Dove prism system is described as the rudi- 

 ment of the prism binocular or shortened telescope. 

 Such a system, unlike the earlier Porro combination, 

 cannot be placed in the convergent beam and it cannot 

 serve to shorten the telescope. 



Those to whom style and composition are of im- 

 portance will regret the inelegance of such expressions 

 as " credulous twaddle," " pricked" up its ears," and 

 NO. 2767, VOL. I IOJ 



" blast of hot air." Their use detracts from the 

 pleasure of perusal of a welcome addition to the 

 literature of the telescope. James Weir French. 



An Elementary Work on Coal-Mining. 

 An Elementary Text-Book of Coal-Mining. By Robert 

 Peel. Revised and enlarged by Prof. Daniel Burns. 

 Twentieth edition. Pp. viii + 420. (London and 

 Glasgow : Blackie and Son, Ltd., 1922.) 6s. net. 



THIS little book is, as its title expresses, a book 

 dealing with the most elementary principles 

 of coal-mining. It has obviously answered its purpose 

 extremely well, and has suited the needs of those to 

 whom it is particularly addressed, as is only too evident 

 from the fact that it has reached its twentieth edition 

 since its original publication twenty-nine years ago. 

 It need scarcely be said, therefore, that the general 

 arrangement and style of the work are beyond criticism, 

 otherwise it would not have survived the rigorous test 

 of experience through which it has passed. Any 

 review of the work must therefore be based upon the 

 nature of the revision to which it has been subjected. 



It may fairly be said that the labour of revising such 

 a work falls under three main heads, namelv, first to 

 eliminate all possible blunders ; secondly, to bring the 

 work thoroughly up-to-date, and thirdly, to see that 

 there is no ambiguity likely to puzzle the student. 

 Unfortunately, it cannot be said that the revision stands 

 the test under any of these three heads, and a couple 

 of illustrative examples of shortcomings may be quoted 

 under each. There are, for example, blunders in 

 spelling, such as " Plainmeller " for v Plenmeller " and 

 " Maudline " for " Maudlin." Under the second head- 

 ing we have such statements as that the deepest bore- 

 hole in the world is that at Schladebach, which attained 

 the depth of 956 fathoms. This was true once, but the 

 deepest borehole in the world to-day is that at Czuchow, 

 Rybnik, Upper Silesia, which has attained a depth of 

 7350 feet. Again, the statement that of centrifugal 

 fans those most generally adopted are the Guibal, 

 Waddle, and Schiele was true once, but is not true to-day. 

 The only reference given to the Kind-Chaudron method 

 of sinking in this country is its first application at 

 Marsden, the far more important, instructive, and recent 

 sinking at Dover not being mentioned. Under the 

 third head we get such a statement as that when it is 

 inconvenient to state work in foot-pounds as the unit 

 of work a higher unit is adopted termed horse-power. 

 The confusion between work and power, to which most 

 students are prone, is one that should never be allowed 

 to creep into a text-book, where the difference between 

 the two standards should be very clearly explained. 

 Again, in dealing with the thickness of tubbing, two 



