March i6, 191 6] 



NATURE 



67 



Messrs. John Wheldon and Co., 38 Great Queen 

 :>:reet, Kingsway, W.C., have just issued a catalogue 

 of important books and papers on cryptogamic botany 

 they are oflfering for sale. The works are arranged 

 conveniently under three main divisions — economic, 

 geographical, and general — each of which is subdivided 

 to facilitate search for works on any particular sub- 

 jects embraced by the catalogue. 



OVR ASTRONOMICAL COLUMN. 



Comet 1916a (Neljmin). — Copenhagen Postcards 

 Nos. 13 and 14 give orbits and ephemerides for this 

 comet calculated by M. J. Fischer-Petersen and Mile. 

 J. M. Vinter-Hansen. The earlier orbit is based on 

 observations made at Yerkes (Februan,- 29), Green- 

 wich (March i), and at Bamberg on March 3. The 

 second, given below, depends on the Yerkes and Bam- 

 berg positions, and observations made at Bergedorf 

 on .March 5 : — 



Perihelion Passage (T), 1916, March. 9-417 G.M.T. 

 a> =191' 9' 87"! 

 ^1=325° 24' io"li9i6-o 



z = 16" r 48"; 



log g =01 9036 



R.A. Dec. R.A. Dec. 



n. m. s. „ , h. m. s. . 



-March 15 9 5 414-5 58-2 March 21 9 10 56 + 3 in 

 17 7 17 5 i"2 23 12 57 2 i8-i 



19 92454 25 15 6 I 267 



The orbit is apparently periodic in short period. 

 The comet is fainter than ii-o mag. 



Co.met 19156 (T.WLOR).— A new orbit and ephemeris 

 for this comet has been calculated by M. J. Braae 

 from observations made at Rome, December 5, 1915, 

 at Arcetri and Copenhagen, Januar^• 11, 1916, and at 

 Bamberg and Copenhagen on February- 20. The new 

 orbit only differs slightlv from the earlier elliptical 

 orbit (Nature, January 20) : — 



Perihelion Passage {T)^i^i6, January 30-9122 G.M.T. 



Epoch 1916 Jan. 05 G.M.T. Equinox 1916-0 



-^lo =355° 17' 34'6" (0 =354' 47' 54-9" 



M =557";i9i a =113 53 57-6 



Log 0=0536002 /• = x5 31 510 



U =232595 days (^ = 33 7 33-9 

 (6-37 years). 



. The comet is very weak, having been about 12 mag. 

 on February 20. 



From Bergedorf, Prof. Schorr has reported (Circular 

 'Y' 503, Astronomische Nachrichten) that the nucleus 

 of this comet has divided into two portions. The 

 nuclei were of unequal brightness, about magnitudes 

 II and 13. Their positions were :— Distance, 14"; 

 position angle, 169° and ij" and 25°, on Februarv 19 

 and 29 respectively. On the latter date the following 

 nucleus was the weaker. 



According to a note in the current number of the 

 Observatory, Prof. E. E. Barnard observed the double 

 nucleus on Februar}' 9, the separation being 10'. 



Variable Stars in the Vicinity of R Coron^e 

 AusTRALis.— This region is under careful scrutiny, not 

 only at Hehvan, but also at the Union Observatory, 

 Johannesburg. In Circular No. 31 both R Coron'se 

 and the nebula are stated to be variable over a wide 

 l^nge. The observations of these objects are to be 

 discussed later. Thirtv-three new variable stars have 

 been detected in the region. 



A Possible Deflection of Light by a Moving 

 AlEDiLM.— Prof. P. Zeeman has published {K. 

 NO. 2420, VOL. 97] 



Akademie van Wetenschapen, vol. xviii., pp. 71 1-5/* 

 an investigation of the propagation of light-waves 

 along a velocity gradient in a moving medium speci- 

 ally in relation to solar phenomena. From a con- 

 sideration of the Lorentz. dispersion term in the Fres- 

 nel coefficient, it is demonstrated that the simultaneous 

 existence of velocity gradients and anomalous dis- 

 persion in gases that are extremely rare {e.g. the 

 absorbing vapours giving rise to the finest lines in 

 the solar spectrum), and without density gradients, 

 may give rise to a deflection of light. 



.1 TLNGSTEN TARGET FOR X-RAY 

 TUBES. ^ 



GREAT advances have recently been made in the 

 production of X-rays, chiefly by the employment 

 of very heav\- currents. The expKJSures necessary for 

 producing radiographs of the thorax have been reduced 

 from minutes to fractions of a second. 



To make this possible, much attention has been 

 devoted to the target or anti-kathode, which is the 

 critical part of the tube, for here it is that the focus 

 of the kathode stream strikes, and the energy of the 

 bombarding electrons is transformed into X-radiation. 

 The early English tubes were furnished with sub- 

 stantial targets of platinum, but in the later foreign 

 tubes with which the market was flooded the platinum 

 was often reduced to a sheet of very thin foil laid 

 upon a plate of nickel. For weak currents, and with 

 an imperfectly focused kathode stream, this plan 

 answered moderately well, but if heavy currents were 

 used the heat generated at the focus was often so 

 great that the platinum skin alloyed with the nickel 



backing, when 

 fusion and de- 

 struction of the 

 whole apparatus 

 followed immedi- 

 ately. 



This is well 

 illustrated by the 

 accompan ying 

 photograph 

 of such a fused 

 target which ap- 

 peared some time 

 ago in the Journal 

 of the Rontgen 

 Society. 



Recently atten- 

 tion has been 

 directed to the 

 exceptional pro- 

 perties of pure metallic tungsten, now pro- 

 duced in quantity for the manufacture of metal 

 filament lamps, and its suitability for the purpose was 

 at once recognised, the metal having a fusing f>oint of 

 about 3000° C, as against 1750° C. for platinum. 

 Tungsten is also very tough, and does not readilv 

 disintegrate by the kathodic discharge (kathode sput- 

 tering) ; its atomic weight, 180, is not much below 

 that of platinum. 



The British Thomson-Houston Company, Ltd., has 

 introduced a sf>ecial target of this metal that is being 

 largely used by manufacturers of X-ray tubes. The 

 tungsten is in the form of a thick button brazed into 

 a solid block of copper, in some cases weighing as 

 much as half a pound ; this forms a lasting and 

 efficient target, even when heav\' currents are used 



1 " Quantitative Meaiurements of the Conversion of Kathode Rays into 

 Rdntgen Rays by Anti-kathodes of different Metals." By J. H. Gardiner. 

 Journal of the Rdotgea Society, No. 24, vol. vi. 



Platinised nickel target damagtdby the kathode 

 focus. 



