72 



NATURE 



[November 17, 1910 



relationship is less obvious than usual, because the 

 fungus only fructifies on the tree that it has killed, 

 and the mycelial threads are only discoverable with 

 difficulty on the livinp^ wood. The proof lies in the 

 fact, well brought out in this report, that inoculation 

 of a healthv tree with a piece of the fungus nearly 

 always causes the disease. 



As its name implies, the disease is characterised by 

 the silvery look taken on by the leaves, due, appar- 

 entlv, to a disconnection of the cells. Changes in 

 nutrition processes are, no doubt, the immediate cause, 

 but it is sufi^gested that the final cause is a poison 

 formed during the growth of the fungal threads, 

 which then spreads into the tree. This hvpothesis 

 explains, amonp- other things, whv the fungus is never 

 found on the diseased leaves. The disease is usually 

 fatal. All kinds of fruit are not equally susceptible ; 

 plums come first, followed by apples, laburnums, Por- 

 tugal laurels, and pears as the least susceptible. If a 

 tree recovers, it mav still be badly attacked again ; 

 there was nothing to show that previous infection 

 tends to immunise trees against subsequent attacks. 



In New Zealand, where this disease is also trouble- 

 some, the application of ferrous sulphate is recom- 

 mended as a remedy, but the authors cannot find that 

 it is of any value. Indeed, no method of treatment 

 seomed trustworthy, and all that the grower can do 

 a.'j yet is to burn afTected trees and so prevent the 

 disease from spreading. 



'Elementary Treatise on Physics, Expetimcntal and 

 ApHied, for the Use of Colleges and Schools. Trans- 

 lated from Ganot's " Elements de Physique." By 

 Dr. E. Atkinson. Eigfhteenth edition, edited by 

 Prof. A. W. Reinold, F.R.S. Pp. xiv+1225. 

 (London : Longmans, Green and Co., 1910.) Price 

 155. 



All teachers and most students of physics know 

 Ganot's " Physics," and will be interested in the 

 appearance of another new edition. On examining 

 the new volume they will find changes in the 

 arrangement of subjects and chapters. In the section 

 dealing with heat, the subjects of solution, equili- 

 brium, and liquefaction have been put into separate 

 chapters. Radiation is now dealt with under light. 

 In numerous parts -of the book extensive additions 

 have been made, and much new matter on modern 

 subjects of physics of g-reat importance has been 

 added. But to prevent an undue increase of size, 

 sections dealing- with matters of no interest to 

 students of to-day have been omitted. In its new- 

 form the treatise is likely to continue its popularitv. 

 When another edition is necessary the editor should 

 substitute a modern form of rain-gauge for that on 

 p. 1146, and revise the section on the Gulf Stream on 

 p. 1 172, where several time-honoured fallacies are 

 repeated. 



Dunkelfeldbeleuchtung und Ultramikroskopie in der 

 Biologie und in der Medizin. By N. Gaidukov. 

 Pp. vi + 8^+Tafel v. (Jena: Gustav Fischer, 1910.) 

 Price 8 marks. 



This booklet g-ives a brief but fairlv complete sum- 

 mary of the researches which have been pursued by. 

 means of dark g^round illumination and the ultra- 

 microscope in the domain of biology and medicine. 

 Thus the structure of colloids and of "sols" and 

 "gels" and the minute structure of various animal 

 and vegetable cells as revealed bv these methods are 

 epitomised. But the methods themselves receive but 

 the scantiest notice, the theory of the subject and th-e 

 apparatus being dismissed in the space of a couple 

 of pages. Those who desire to work at the subject 

 will therefore have to seek instruction elsewhere. In 



NO. 



2142, VOL. 85] 



some cases we do not think justice is done to ordinary 



methods of illumination ; in Fig. 5, for example, a 

 comma bacillus is depicted as being practically 

 structureless when viewed by transmitted light, 

 whereas with care a certain amount of structure can 

 alwavs be made out. Dark ground illumination no 

 doubt does much to elucidate the finer structure of 

 minute unicellular organisms; how far the ultra- 

 microscope will help remains to be proved. 



The work concludes with a very useful bibliogfraphy. 

 and is illustrated with numerous fi^j^ures in the text 

 and five plates, two of which are coloured. 



R. T. H. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 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. 

 .Vo notice is taken of anonymous communications.] 



The Limiting Line of Sedimentation in Wave-stirred 

 Areas. 



If you can spare the space I think I can put your 

 reviewer (October 6, p. 433) in the way of obtaining th>' 

 information he seeks as to the " limitinj^ line of sedi- 

 mentation " in " wave-stirred areas." The presence of 

 tidal and other currents is assumed. 



In a paper to the Royal Society in 1882 I chanced to 

 hit upon this limit, experimentally and incidentally, in the 

 following observation : — 



" Dried peas placed on a glass plate in a slight 

 depression on a sandy bottom in 6 inches of water were 

 rolled off by waves about 12 inches long and about i inch 

 high. . . . 



" Shorter w-aves ih inches high had much less effect on 

 them. A little sand that had collected on the glass was 

 beautifully rippled with |-inch ripples " (Proc. Roy. Soc, 

 1882, p. 16). 



According to this chance experiment the limit was rather 

 more than half the wave-length. 



In 1884 I submitted to the late Sir G. G. Stokes, F.R.S. . 

 the case of a soda-water bottle, trawled at about 40 

 fathoms in the English Channel, which exhibited evidence 

 that It had been subjected to long periods of quiescence, 

 with intermittent disturbance. Sir George Stokes replied 

 in the very important letter published in my paper on th.- 

 Skerries Shoal (Trans. Devon. .Association, 1887, p. 498). 

 For publication in the same paper I had asked Lord 

 Rayleigh to give me some simple formula for ascertainin|:j 

 the practical limit, of wave-action. His reply was: — 



" For each step downward of A/8 divide by 2-2." I 

 mav mention that A represents the wave-length. 



Now according to this formula the disturbance at a 

 depth of half (or four-eighths) of the w-ave-length is about 

 one-twenty-third of that at the surface, whereas at the 

 depth of five-eighths it is about one-fiftieth. I believe that 

 one-fiftieth is negligible, whereas one-twenty-third is not 

 always so. 



Thus the limit of disturbance lies between half and five- 

 eijjhths of the wave-length. 



This exactly agrees with my accidental tank experiment. 



For further confirmation I may refer your reviewer to 

 Stevenson's interesting discussion on the " Level assumed 

 by Mud a Measure of Exposure " (Stevenson on Harbours, 

 second edition, p. 16). 



I will not trouble your reviewer with my own papers 

 except to mention one in the Linnean Society's Journal, 

 Zoology, vol. xviii., p. 263, " On the Influence of Wave- 

 currents on the Fauna inhabiting Shallow Seas." 



.'\t the recent inquiry on coast erosion Mr. R. H. Worth 

 cited a delightful zoological proof of a local limit of dis- 

 turbance. Speaking of Alcyonium digitatum and several 

 other hydroids, Mr. Worth stated that : — 



" Somewhere below 35 fathoms they will attach them- 

 selves to light bodies ; above 25 fathoms they will never 



I 



