494 



NATURE 



[February 20, 1919 



utilitj ol the vectorial method of raj tracing, and 

 the desirability oi further information 01 

 this point. Dr. Silberstein writes : -" . . . In ordei to 

 help the spread and the easj handling of the vei ■ 

 method, in the spirit of Dr. Brodetsky's closing sen 

 tence, I shall be glad to do personally .ill in m\ powei 

 in removi doubts and apparenl difficulties. In this 

 respect half an hour's personal conversation is certaii 

 to be more efficient than man; hours dedicated to the 

 writing ol notes 01 papers foi publication. The formei 

 has, moreover, thi obvious advantage ol being adapt- 

 able to the individual needs ol the questioner. In 



in meet, in pari at least, these needs, I gladl) 

 offer myself to give free information on the subject in 

 question to everybod) ivho will care to call personally 

 (not In letter.) .11 1 Vnson Road, Cricklewood, London, 

 N.W.j, where I shall be available for that purposi 

 on every Friday from 5.30 until 7.30 p.m." VVe hi 

 much pleasure in making public Dr. Silberstein's offer, 

 and feel that sunn ill our readers will gladlj avail 

 Mi in-i Ives nf this unique opportunity of being initiated 

 into the practical application of vector methods by a 

 master of the subject; At the same time we suggesl 

 that Dr. Silberstein would be doing a service to a 

 wider circle of those interested in optical work if he 



11 publish one or two detaili d 1 omputations bast d 

 mi his formula?. 



I in Following works are in the press for publication 

 by the Carnegie Institution of Washington: "The 

 Duration of the Several Mitotic Stages in the Root- 

 tip Cells ol the Onion," II. II. Laughlin; "Con- 

 tributions t.i the Genetics of the Drosophila melano- 

 r," T. II. Morgan, C. B. Bridges, and A. II. 

 Sturtevant; "The Genetic and Operative Evid 

 Relating to Secondary Sexual Characters," T. II. 

 Morgan; and "Studies of Heredity in Rabbits, Rats, 

 and Mi,,," VV. E. Castle. 



OUR ASTRONOMICAL COLUMN. 

 Luminosities \\n Distances of Cepheid Variables. 

 — In continuation of his important studies of stellar 

 clusters, Dr. Harlow Shapli \ has investigated the 

 luminosities, distances, and distribution of the Cepheid 

 variables (Astrophys. Journ., vol. xlviii., p. j7.1i. 

 Restricting the discussion to variables with definitely 

 determined periods of less than forty days, there 

 are forty-five stars which are of the ■■cluster'' typt 

 and ninety-four ordinary Cepheids with periods greater 

 than a day. The absoluti magnitudes and parallaxes 

 have been determined bj means of the luminosity- 

 period relation, with an average probable error esti- 

 d at jo per cent. The cluster-type variables are 

 found to have absolute luminosities a little more than 

 one hundred times thi brightness of the sun, while 

 the ordinary Cepheids range from two hundred to ten 

 thousand times that of the sun. Fewer than one-third 

 of the st.n-s have parallaxes greater than a thousandth 

 of a second, and the most distant Cepheids now known 

 are nearly 20,000 light-years from the sun. While the 

 Ordinarj Cep are strongly concentrated towards 



thi galactic plane, the cluster-type variables are in- 

 different to that plain. The wide dispersion of the 

 latter may probab unted for by their rela- 

 tively high velm ities in 



Radial Velocities of no Stars.— A preliminary 



1.11 .nil! nt tin 1 adial > eloi of 1 [9 stars, as deter- 



mined at the Cape Observatory, has been given b] 

 Dr. J. I.uni (Astrophys. Journ., vol. xlviii., "p. 161) 

 The number ol thesi stars which probably have con- 

 stant velocities is seventy-six, while the remaining 

 hrei an eithei know n 01 suspi 1 ted spi ctro- 

 binaries. Eighteen of thi stars in the first 



-'^73- vol. 102] 



class wen ven frequently observed in connection with 

 the spectroscopic determination of the solai parallax, 

 the total number of plates obtained for them ! 

 552. I In- lull,, win- are among the results foi some 

 of the bright stars, as compared with thi 

 obi d at the I ,ick < >bsei vatoi j ; 



lociiy 



St.,, 



km. L-m 



a \i ietis 



a Tauri 



a ( an. Min. 



S 1 ieminorum 



a I [ydrae 



. Virginis ... 



a Bootis 



a Serpentis 

 1 Sagittarii 

 a Aquarii 



Approach to the sun is indicated by a minus and 

 1 1 1 essii hi i'\ a plus sign. 



"Anuario del Observatorio de Madrid."— This 



useful annual for iqiq contains all I he customary 

 astronomical data, including the times of rising and 

 setting of the moon (which might with great advan- 

 tagi In inserted in our own Nautical \lnianac). There 

 are also several essays; one, bj A. Vela, givi 

 r&suml ol researches on the temperature of the sun's 

 photosphere, concluding in favour of 7000 . C. Pu< nt< 

 shows how to find time and latitude from the observed 

 altitudes of two stars; this can be solved graphically 

 by the well-known Sumner method. Dr. F. Iniguez, 

 1I1. director of the observatory, gives an interesting 

 monograph on Nova Aquilse, with photographs of the 

 spectrum from June 9 to September 4, and a light- 

 curve, which appears to show that the period of varia- 

 tion was about twelve days in July, but more than 

 a month in August and Septembi 



Ven full details are given ol the sun-spots and 

 prominences observed at Madrid in 1017; also the 

 results of observations of solai radiation between 

 1 11 1 7 September 1 and 1918 August 31. 



The remainder of the volume is occupied by the 

 meteorological observations of 1917. 



THE CHEMISTRY OF SEAWEEDS. 



""yHE scarcity of potash compounds, of iodine, and 

 ' of foodstuffs caused by the great war has 

 directed increased attention to seaweeds during the 

 past four years, and to the possible extension of the 

 use of these as a source of such materials. For some 

 years before the war the giant seaweeds of the Pacific 

 Coast were the subject of systematic investigation in 

 the United States, especially with a view to their 

 utilisation as a source of potash. After the outbreak 

 of war, when many countries, including the United 

 Slates and the countries allied against Germany, wen- 

 cut off from their usual supplies of potash compounds 

 from the German mines, examination began to be 

 made of all sources from which potash might be ob- 

 ained independently of Germany, and seaweeds came 

 in for an increased amount of attention. 



If we consider the great supplies of seaweed which 

 are available, especially in the case (if an insular 

 countn like our own, with a long and deeply 

 indented coast-line, it is remarkable how little has 

 In . n dune, either from tin- purely scientific or from 

 the industrial point of view, for the thorough and 

 matic exploration of the chemistry ..f seaweeds. 

 \ criticism be Prof. C. Sauvageau, 1 ol Bordeaux, of 



■ or Irs Analyses ChiWi 1 



No. 19, October, 



