2b6 



NATURE 



[SErTEMBER I, I9IO 



(i) A First Year's Course of Inorganic Chemistry. By 



G. F. rtood. Pp. iv+107. (London: Rivingtons, 



igio.) Price is. 6d. 



(2) .1 Manual of Elementary Pnietical Chemistry for 



Use ill the Laboratory. By P. W. Oscroft and 



R. P. Shea. Pp. viii + 134. (London: Rivingtons, 



1910.) Price 2S. 



These two little volumes are for use in schools, and 



are intended to serve as an introduction to chemistrv. 



Oscroft and Shea's manual carries the subject to the 



stage of equivalent weight estimations and simple 



gravimetric and volumetric analysis, whilst Hood's 



book, which is a first year's course, stops short of 



this point. Both books contain descriptions of a 



series of easy quantitative experiments on loss and 



gain in weight, as well as a detailed account of a 



variety of thoroughly instructive preparations. 



There is nothing; in either that strikes one 

 as very new or original in conception or 

 arrangement; but, on the other hand, there 

 is nothing to which objection can be taken, and 

 both volumes mav be recommended without 

 reservation. It might be well in a future issue to 

 give the actual results of the quantitative experiments 

 so that both teacher and student might form some 

 idea of the accuracy attainable. In conclusion, we 

 question to what extent it is permissible to adapt a 

 classical discovery to the intelligence of a schoolboy ; 

 for it may be doubted if either Berthollet, Gav-Lussac, 

 or Davy ever thought of chlorine as '' murium 

 dioxide" (Hood, p. 51). J. B. C. 



Catalogue of the Books, Manuscripts. Maps, and 

 Draivings in the British Museum {Natural History). 

 Vol. iii. (L-O). Pp. iv+ 1039-1494. (London: 

 British Museum (Natural History), 1910.) Price 20s. 

 The long interval which has elapsed since the pub- 

 lication of the second volume of this catalogue (see 

 N.VTURE, August 25, 1904)— which followed the first 

 (ibid., October 22, igoj) in reasonable time — is ex- 

 plained in the preface as due to other librarv 

 work. Apparently the earlier sheets of this 

 volume were completed and printed off before 

 1907, as we find no title associated with the 

 name of Sir E. Ray Lankester, while the latest of his 

 works referred to bears the date of 1906. In this con- 

 nection it may be noted that in some cases the full 

 Christian names of authors, as in the case of Sir 

 E. R. Lankester and Sir R. Owen, are repeated in 

 each entry, whereas in other instances, as in the case 

 of Sir Charles Lyell, these are reduced to the initials 

 after the first entrv. Apparentlv the compiler 

 was compelled to follow the order' adopted in the 

 library catalogue at Bloomsbury, which will probably 

 account for the sundering of such names as Loenn- 

 bohm fp. 1 163) and Lonnberg (p. 1175). Like its 

 predecessors, this volume contains valuable biblio- 

 graphical information, and it is to be hoped that we 

 shall have the pleasure of welcoming the fourth 

 volume at an early date. R. L. 



The Calendar of Garden Operations. New and en- 

 larged edition. By members of the staff of the 

 Gardener's Chronicle. Pp. vi4-i75. (London: 

 Gardeners' Chronicle, Ltd., 1910.) Price 6ii. net. 

 This is a new edition of a work prepared originally 

 by Sir Joseph Paxton, and published in 1S42. It is 

 a concise and practical manual from which possessors 

 of small gardens in country or town mav obtain 

 much useful advice and guidance. Chapters have 

 been added on the cultivation of trees and shrubs in 

 towns, and on the principles of intensive culture or 

 French gardening. In its enlarged form the con- 

 tinued success of the book is ensured. 



NO. 2 13 I, VOL. 84] 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opiniorts 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



Separating Power of a Telescope. 



Can an observer inform me what are the proper tele- 

 scopic powers, and apertures or sizes of glasses, required 

 to see stars which are apart from each other the follow- 

 ing distances, and of different magnitudes? 



(i) ... o to I (5) ... 8 lo 12 



(2) ... I 10 2 (0) ... 12 10 16 



(3) ... 2104 (7) ... 16 to 24 



(4) ... 4 to 8 (8) ... 24 to 32 

 Gore made a table showing the magnitude of the faintest 



star visible in any telescope in his " Stellar Heavens," but 

 I would like to see if anybody can make a table out 

 from experience and ordinary practical observation in the 

 case of double and multiple stars. Where the glare of one 

 star interferes with the definition of another star, I am 

 inclined to think a larger aperture is needed in the case 

 of doubles and multiples than in singles, and where the 

 stars are very close. If a table could be made out for 

 reference by students it would be useful. 



Grimscar, Huddersfield. J. W. Schoi.es. 



The questions proposed cannot be answered quite so 

 definitely as Mr. Scholes would appear to think. To begin, 

 we must have a clear idea of what is meant by the 

 separating power of a telescope. Put in a theoretical 

 form, it means : Given two points of light (stars) a certain 

 angular distance apart, wliat is the size objective which 

 will just give two distinct images in the focus? For 

 practical purposes, this is answered by the formula 



o .■ 4"''i6 



SepiraUng power= — -^ 



where a is the aperture of the object-glass in inches. 



This can easily be remembered ; or, if it is preferred, a 

 table can readily be formed, thus : — ■ 



of O.G. Separating power 



Inches 



456 



S 0-57 



28 0-17 



36 °'i 



A second of arc in the focus of the 28-inch Greenwich 

 refractor is ©•ooi63 inch. Let it be clearly understood that 

 this table gives the theoretical size of object-glass to obtain 

 separate points in focus. 



If the points are not separated in the focus, no amount 

 of magnifying power will afterwards separate them. 



This does not imply that the unaided eye, looking at the 

 focal images in the telescope, can detect duplicity. The 

 points to be thus seen must subtend at the eye an angle 

 of at least 60". It is here that the eye-piece comes in, 

 for, given two separated points, we can magnify the separa- 

 tion until the eye can not only detect, but can sec it 

 sufficiently to enable micrometric measures to be made. 



Hence we use eye-pieces magnifying 2, 3, 4, 5 times, 

 and so on. 



These eye-pieces bring up the total magnifying power, 

 which is quoted as the power used. It is stated in text- 

 books that a power of 50 or 60 per inch aperture is the 

 practical limit ; but it is readily seen, however, that the 

 power used is dependent on several conditions : — 



(i) Size of object-glass. 



(2) Quality of object-glass. 



(3) Condition of the atmosphere. 



(4) Personnlitv of the observer. 

 (0 Subjr,! ,,r,srrved. 



The i.|U' siiiiiis :ivk<cl refer especially to double-star work, 

 and conliiiiiii: (iuim Iv.-s to this simplifies matters. 



If we alicnv s" pir inch aperture, we should expect to 

 llnil ohservei-s with the 



