652 



NA 1 URE 



[November 5, 1923 



convenient 

 purpose as 

 that 11 

 be tli'ii 

 them I 

 gauge, 

 that tla 

 did not 



ni.i 



OI' 



re 



.sUil 



^aiiK*" even UiX stich a con^ 

 <1. trcting leaks and ensuring 



IS in good order. We think u« 

 . rvicc to our colleagues if we ur;. 

 . I ages for this purpose of the In ; i 

 '.\ if used according to the nut hi 

 of these laboratories described Omu 

 liscover) in Physical Society Procc^eding.s 

 vol. 33, p. 287, 1921- 



The Krcat advantage of this instrument is its 

 * npHcity. In addition to some very 

 ,ii gear — a battery, rheostat, 3 fixed 

 > . .... .uljusted very roughly, a cheap pointer 



galvanomcttr, and a respectable voltmeter — it needs 

 nothing but an ordinary incandescent vacuum lamp. 

 Since lamps are cheap and since the same electrical 

 gear will serve any mmiber of lamps, there is no limit 

 to the number of gauges which can be readily attached 

 to the same piece of apparatus. The diagnosis of 

 leaks and other faults is a very simple matter when 

 gauges are attached at almost every joint, and their 

 readings with the pump running are compared. Hut 

 this is not its only virtue. We are certain that any 

 one who tries the Pirani gauge will forthwith consign 

 his McLeod gauge to the dust heap and wonder how 

 he ever managed with such a cumbrous and mis- 

 leading device. 



It is sometimes objected to all gauges but the 

 McLeod, that their calibration depends on the nature 

 of the gas. To this we would reply that in every 

 experiment we can imagine in which a knowledge of 

 the absolute value of the pressure is required, either the 

 nature of the gas is known or it has to be determined 

 for some purpose other than that of reading the gauge. 

 As we have said, an ordinary incandescent lamp 

 will do as a gauge ; but it is even simpler (and 

 for various reasons preferable) to use the same lamp 

 before it is evacuated and with the pumping stem 

 still attached. Probably any lamp maker would 

 supply such lamps ; if they are obtained from these 

 laboratories, they will be furnished with a rough 

 calibration — a calibration as good as that which the 

 McLeod usually receives. 



Norman R. Campbell. 

 Bernard P. Dudding. 

 John W. Ryde. 

 Research Laboratories of the 

 G.E.C., Ltd., Wembley. 



Zoological Bibliography. 



I AM desired by the Corresponding Societies' Com- 

 mittee of the British Association to direct attention 

 to the-Report of the Committee on Zoological Biblio- 

 graphy and Publications, which was presented at 

 the Liverpool meeting of the Association, and to 

 ask those interested in the publications of scientific 

 societies earnestly to consider the recommendations 

 made by this Committee, and thus avoid the un- 

 necessary confusion and difficulties which arise from 

 thoughtlessness rather than ignorance. 



There are many important points to be borne in 

 mind, particulars of which can be seen in the Report 

 of the Committee, which can be obtained from the 

 Secretary of the British Association, Burlington 

 House, Piccadilly, W.i, but those to which par- 

 ticular attention is desired are : (i) The size of 

 the publication, which should be demy-octavo (that 

 is, the size of the Reports of the British Association) ; 

 (2) that each part issued should bear the actual date 

 of publication ; and (3) that the titles of papers should, 

 so far as possible, give a fair idea of the contents of 

 the papers, and be brief. T. Sheppard. 



The Museums, Hull. 



NO. 2818, VOL. I 12] 



A New Method of Crystal Powder Analysis 

 by X-rays. 



rpoHf of enabling us to mak» 



■ r Inifh wilhuut recurring t' 

 i^ement has been tried in \\ 

 powder and a !■■ 



MrKlificatioii 

 and Scherrer aim "i inm k. mi ^^I(lt• 



have been described by IL by H. I 



by Sir William Hragg, and \^^ i... filler.* It 

 particular been shown by Sir William Bragg, t; 

 Ijis arrangement, which involves the use of the u.... ,.. 

 tion method, it is possible to make not only rapi<l 



Fig. 



but also very accurate determinations. The present 

 arrangement is intended to correspond to the peculiar 

 conditions of the photographic record. 



A short reference to the general c-"-'''''^"-^ "* *■■ 

 flection may take the place of an extc: 

 The geometrical locus of all cr>"stal pi — .. . , ... .....^ ,, 



which are so situated that rays reflected by them 

 from A to C (Fig. i) suffer the same deflexion a, is 

 that surface of revolution described by the rotation 

 of the arc of a circle .\BC subtending the angle x - a 

 on the chord AB. This surface has a different shape 

 for each angle of deflexion a. 



To obtain simple conditions for a quantitative 

 interpretation of the reflected intensities the writer 

 had used {he. cit.) an equatorial annular band of this 



Fig. 3. 



surface in conjunction with a point source of X-ra}"^. 

 At present in order to obtain lines which are more 

 suitable for exact angular measurement, only a small 

 area round B is used in connexion with a line source. 

 The line source allows ns to make more eflScient use of 

 the radiation of the anticathode and partly compen- 

 sates for the decrease in angular extension of the beam. 

 Fig. 2 represents the arrangement adopted. A is 

 the line source of X-ra>'s situated close to the anti- 

 cathode, B the powder layer, which can be rotated 

 about an axis parallel to the source, and C is the film 

 on which the lines are recorded. According to the 

 geometrical relations giv^en above, to every angle of 

 deflexion, i.e. to every point on C, there is associated 



» H. Seemann. Ann. d. Pkys. 59, PP- 455464, 1919; H. Bohlin, Atm. d. 

 Pkvs. 61, p. 421, 1920; Sir Wiiliara Bragg, Proc Phys. Soc 33, p. 222, 

 1921 ; J. Brentano, Arch. Sc. Pkys. ei Sat. (5) i, p. 550, 1919. 



