6o6 



NA TURE 



[Oct. 23, 1884 



with which he, even their own author, accepts such 

 startling speculations concerning the hereditary nature of 

 the parasitic diseases of plants. 



In the matter of the potato disease, Mr. Smith gives a 

 history of the whole subject, and a full description of the 

 oospores, which he claims to be those of the Phytophtlwra. 

 At p. 340 there is a sentence of some interest in view of 

 the above-mentioned theory. 



" It is quite possible, then, that just as every atom of a 

 mycelial thread of this fungus (potato fungus) will continue 

 its growth to a perfect form, so every atom of a broken- 

 up nagellum — perfectly invisible to the eyes even when 

 the highest powers of the microscope are used — may be 

 capable of carrying the poison and at length reproducing 

 the perfect form of the fungus in the potato plant." 



Everything is possible, but some things are undoubtedly 

 highly improbable, and chief among these are those which 

 we have not the slightest grounds for supposing probable. 

 Such is the case with this speculation since (to take the 

 nagellum only) in the first place it is not by any means 

 certain, as the author indeed points out in the same para- 

 graph, that a nagellum breaks up at all, and in the second 

 it is quite unwarrantable on any known basis of fact to 

 suppose that its fragments are endowed with any repro- 

 ductive function. 



Apart from such speculations, I venture to think that 

 Mr. Smith has rendered the study of vegetable parasites 

 a signal service in the publication of this book. Its 

 practical uses to the farmer and the gardener are appa- 

 rent, and to the student of the subject the advantage is 

 no less, even in those cases where the author differs from 

 the great majority of his fellow-workers, since " the case 

 for the opposition " is as well and as strongly stated as 

 the materials permit. The book is of practical value in 

 this country, and it is, moreover, one which no intelligent 

 agriculturist can afford to dispense with in these times, 

 when farming is engaged in a struggle of such severity at 

 so many points. GEORGE MURRAY 



OUR BOOK SHELF 

 How to Foretell the Weather with the Pocket Spectro- 

 scope. By F. W. Cory. (London: Chatto and Windus. 

 1884.) 

 It is of little use putting any instrument, however simple 

 it may look, into a student's hands, if he is not previously 

 taught how to use it. This needful information is supplied 

 by the handy little book now before us, showing what can 

 be done with a direct-vision spectroscope only some 3| 

 inches long. 



The book commences by describing two pocket spec- 

 troscopes now in use : the " rainband spectroscope," and 

 a newer and somewhat larger instrument, " Grace's spec- 

 troscope," which, however, is still small enough for the 

 pocket, being only 5! inches long when closed, and which 

 has the advantage of giving a larger spectrum. Here, 

 however, there is a most important omission, for the 

 adaptation of a lens to focus the image of a cloud or a 

 part of the horizon on the slit is not referred to. Instru- 

 ments thus armed are far better than those of the ordinary 

 construction for meteorological purposes, and, as made 

 by Hilger, they are not appreciably larger. We are next 

 told how to use the spectroscope, and a map is given 

 (Plate 1), showing the positions of some of the lines 

 which the student should learn to recognise in the spec- 

 trum of the sun, in order to see at once if the rainband 

 is present or not. 



On another page we find the principal rainband itself 

 (Plate 2), which is instructive as showing the student 

 what to look for ; but in the construction of this map a 

 larger spectroscope, of two prisms, has been employed, 

 so that if the student in looking for the rainband uses his 

 pocket spectroscope, he will be somewhat disappointed. 

 It would have been more complete if a drawing of the 

 rainband, as seen with Grace's spectroscope, could have 

 been given side by side with Plate 2, which shows so 

 much of the detail. 



The book concludes with letters, reprinted from the 

 Times, from the Astronomer-Royal for Scotland and 

 others, showing the value of the spectroscope for 

 meteorological purposes. 



We think no one can lay down this little volume with- 

 out feeling this opinion confirmed, and that in the pocket 

 spectroscope we possess an invaluable instrument with 

 which to forecast the state of the weather. B. 



Celestial Motion: A Handy Book of Astronomy. By \\". 



T. Lynn. (London: Stanford, 1S84.) 

 Mr. LYNN'S long training at the Royal Observatory has 

 eminently qualified him to write this little book. It is in 

 no sense a school-book, but all the same it contains a 

 most useful introduction to those parts of the science of 

 astronomy of which it treats. These are the earth, sun, 

 and moon ; the planets arranged in three groups ; comets, 

 meteoroids, and the fixed stars. There is added a very 

 painstaking and concise history of astronomical discovery, 

 the only blot in which is an ineffective reference to spectrum 

 analysis at the end. 



The First Six Books of the Elements of Euclid, and 

 Props, i. — xxi. of Book .1/., ond an Appendix on the 

 Cylinder, Sphere, Cone, Sr*c. With copious Annota- 

 tions and numerous Exercises. By John Casey, 

 LL.D., F.R.S. (Dublin : Hodges, Figgis, and Co., 

 1884.) 



This is the second edition of a work which so accom- 

 plished a geometer as Prof. Henrici (vol. xxix. p. 453) 

 has pronounced in these columns to be in many respects 

 an " excellent " book. As the first edition contained 

 254 pages, and this one reaches 312 pages, it is manifest 

 that the work has grown — and with its growth we find 

 that it has acquired an accession of strength. We will 

 indicate in what directions it has increased. First and 

 foremost is the addition of the propositions of Euclid's 

 Eleventh Book, which are generally read by junior stu- 

 dents, and an appendix (well suited for candidates for 

 the London Intermediate Examination) on the properties 

 of the prism, pyramids, cylinder, sphere, and cone. 

 There is also now given an explanation of the ratio of 

 incommensurable quantities, and a still greater number, 

 than in the first edition, of alternative proofs. Further, 

 we can testify, by a careful perusal of the text, that the 

 work has been " thoroughly revised as well as greatly 

 enlarged." One feature we note, that whereas in the first 

 edition the syllabus of the Association for the Improve- 

 ment of Geometrical Teaching was often referred to by 

 quotation, in this edition the name occurs but once or 

 twice. There are reasons for most actions — we presume 

 there are for this course of action. 



We are glad to note that Dr. Casey makes frequent use 

 of the term right line ; the absence of the word " right " 

 is liable to lead young boys astray : we should also prefer 

 in one or two instances the term "circumference" (the 

 line) to the term " circle." 



Numerous easily rectified clerical mistakes occur, and 

 we could wish that the author had uniformly written AB 

 for a line drawn from A to B instead of apparently 

 writing the letters haphazard. The terms area and 

 perimeter are employed without definition ; a work by 

 Prof. Townsend (p. 142) is referred to without giving 

 exact reference ; and an examination question (p. 173) in- 



