April 6, 1905] 



NA TURE 



SM 



the thirty-three years 1871 to 1903, based upon the observ- 

 ations for London (Brixton) published in the daily weather 

 report of the Meteorological Office. The mean annual 

 number of foggy days is 55, of which 45 occurred in the 

 winter half of the year. Dividing the thirty-three years 

 into three equal periods, the result is, for the first period, 

 a mean of 55, for the second 69, for the third only 41. 

 Since the year 1888 a steady and uninterrupted decrease 

 is shown in the mean annual number of fogs. Among the 

 principal agencies which may have conduced to this desir- 

 iible result must be mentioned the efforts of the Coal 

 Smoke Abatement Society and the London County Council, 

 ■also the use of incandescent gas light and electricity ; 

 l)ut, as pointed out by Captain A. Carpenter and Mr. C. 

 Harding, the increase of wind in recent winters is prob- 

 ably chiefly responsible for the decrease of fog. As we 

 have remarked before, the geographical situation of 

 London is, from a purely meteorological point of view, 

 ♦■minently favourable to the development of fog, and the 

 only permanent improvement we can hope for is an abate- 

 ment of its more injurious effects caused bv the imperfect 

 ■consumption of coal and gas. 



We have received a copy of part i. of the " Katalog 

 der Bibliothek der Naturforschenden Gesellschaft in 

 Danzig," published at Danzig in 1904. Although the 

 list of books included is not completely representative, this 

 publication, containing the sections mathematics and 

 astronomy, may be found useful to those desiring to refer 

 to the works of certain authors on these two subjects. 

 The range of subjects is a wide one, and the books are 

 entered under the names of the authors. 



H.'iviNG occasion recently to devise a short-focus spectro- 

 graph. Prof. Wood, of the Johns Hopkins University, 

 found it necessary to make a study of the distribution of 

 light (monochromatic) in the different orders of a typical 

 grating. His method, a beautifully simple one, is de- 

 scribed and illustrated in No. 2, vol. xxi., of the kslro- 

 pliysical Journal. The result showed that, in the typical 

 grating experimented with, half the reflected light was 

 concentrated in one spectrum, and as the grating re- 

 flected about 76 per cent, of the total incident light, this 

 means that about one-third of this total was found in the 

 one spectrum, which was one of the two first orders. It 

 was also found that the ruling makes little or no difTerence 

 to the total reflecting power of the speculum. Two flint 

 prisms of 60° would give about the same average dis- 

 persion as that produced, and, according to Pickering's 

 table in Kayser's " Handbuch," they would transmit a 

 little more than twice the light reflected, in the first order 

 of the grating used. 



The Psychological Bulletin, ii., 2, contains reports of 

 ■the proceedings of the thirteenth annual meeting of the 

 American Psychological Association and of the fourth 

 annual meeting of the American Philosophical Association, 

 which were both held at Philadelphia on December 28-30. 

 Abstracts of the papers are given. Invitations on behalf of 

 Harvard University to hold the next annual meeting in 

 Cambridge, Mass., to signalise the opening of the Emer- 

 son Hall of Philosophy were accepted by both associations, 

 and it is proposed that the Western Philosophical Associ- 

 ation and the Southern Society for Philosophy and Psycho- 

 logy shall also meet at the same time and place. 



\ COLOURED plate of a new species belonging to a new 

 genus of Hydrachnidte is given in the Eendiconti of the 

 Lombardy Institution, xxxviii., 3, in illustration of a note by 

 XO. 1849, VOL. 71] 



Mr. R. Monti on the new " find." This water mite was 

 obtained in cold springs on the right bank of the Anza, 

 near Ceppomorelli, and has been named Polyxo placophora. 

 The same writer in another number of the same journal 

 discusses the horizontal migrations of lacustrine plankton, 

 and finds in mountain lakes that, in addition to the known 

 vertical movements, there are well-marked diurnal migra- 

 tions of the small Crustacea to different parts of the lake 

 depending on sunshine and shade. 



In the March number of the American Journal of Science 

 Mr. Charles S. Hastings utilises some observations of the 

 power of accommodation of the eye for light of different 

 wave-lengths to make a complete determination of the 

 optical constants of the eye for all conditions of accom- 

 modation and for all colours. The results are given in 

 two tables, by the use of which all problems connected 

 with the purely optical properties of the schematic eye 

 may be solved. 



In the course of an investigation of radio-active muds 

 which is published by Prof. G. Vicentini in the Atli of 

 the Royal Venetian Institute (vol. Ixiv., ii., 535), the con- 

 nection existing between the ionisation produced by the 

 mud and the quantity of material used is experimentally 

 ascertained. When the mud is spread uniformly over a 

 definite area, the intensity of the radiation increases as 

 the thickness of the layer is increased, but a direct pro- 

 portionality does not exist between them. After a certain 

 point, moreover, the radio-activity is not increased by 

 adding fresh material. Mr. H. S. Allen, in a paper read 

 before the Royal Philosophical Society of Glasgow on 

 January 25, also deals with radio-active water and mud, 

 the material in this case being derived from the springs 

 of Bath and Buxton. An interesting point which is 

 established incidentally is that the fluorescence excited in 

 a sensitive plate by the radium rays plays only a very 

 minor part in the production by these rays of a photo- 

 graphic effect. 



An interesting investigation of the secondary radiation 

 produced when the 3 and y rays of radium impinge on 

 metallic plates is published by Prof. J. A. McClelland in 

 the Transactions of the Royal Dublin Society (vol. viii.. 

 No. 14). It is shown that the secondary rays are not pro- 

 duced merely at the surface of the plate struck by the 

 primarv rays, but that they come from all parts of a layer 

 of considerable depth. Apparently the less penetrating 

 /3 rays are more efficient in producing a secondary radi- 

 ation than the y or highly penetrating rays. The nature 

 of the secondary radiation depends largely on the character 

 of the metal employed; the greater the atomic weight of 

 the latter the greater is the amount of the secondary radi- 

 ation produced by it. Of all the substances experimented 

 with, lead gives rise to the greatest effect, both as re- 

 gards the quantity of the secondary radiation and its pene- 

 trating power. The secondary radiation consists, appar- 

 ently entirely, of a species of /3 rays, that is, of negatively 

 charged particles capable of deflection in a magnetic field. 

 Perhaps the most important feature of the paper lies in its 

 directing attention to the necessity of considering secondary 

 radiations in all measurements of the absorptive power of 

 substances with regard to the rays produced by radio- 

 active bodies. 



We have received a copy of a memorandum on the con- 

 struction and verification of a new copy of the imperial 

 standard yard, by Mr. H. J. Chaney, superintendent of 

 the Standards Department of the Board of Trade. Since 

 the original standard yard of bronze was made some sixty 



