October 22, 1914] 



NATURE 



199 



THE X-RAY SPECTROMETER. 



TT is now well known that a homogeneous 

 -'■ pencil of X-rays is capable of reflection by 

 a crystal provided that the rays are directed upon 

 the crystal at the proper angfle.^ If .\ is the wave- 

 length of the X-rays, d the spacing of the crystal 

 planes, and 6 the angle which the rays make with 

 the planes, these quantities are connected by the 

 elation 7i\ = 2dsind,^ where n is an integer. 



The object of the spectrometer is to determine 

 the value of 6 in any given case — that is to say, 

 for a definite set of X-rays and a definite set of 

 crystal planes. The results may be classified as 

 follows : If we use different crystals or different 

 faces of the same crystal, but keep the rays the 

 same, we can compare the spacings of the various 

 sets of planes. In this way we arrive at a know- 

 ledge of the relative positions of the atoms in 

 the crystal — that is to say, we determine its 

 structure. 



If we use the same crystal always, but examine 

 the angle of reflection of different homogeneous 

 X-rays, whether from the same or from different 

 sources, we have the means of comparing the 

 wave-lengths of those rays. We can, in fact, 

 analyse X-radiation in exactly the same way as an 

 ordinary spectrometer analyses light. 



The new instrument resembles the ordinary 

 spectrometer in its general construction. To the 

 collimator corresponds a set of narrow slits limit- 

 ing a pencil of X-rays, which is directed so as to 

 pass through the axis of the instniment. A 

 crystal takes the place of the diffraction grating, 

 and is mounted on a small revolving table. The 

 crystal face or set of planes which is acting as 

 reflector is made to contain the direction of the 

 axis of the instrument, and the crystal is turned 

 round the axis until the face makes the proper 

 angle with the incident pencil. The reflected ray 

 then enters a cylindrical ionisation chamber filled 

 with gas which it ionises. The chamber takes the 

 place of the ordinary telescope, and the measure- 

 ment of the ionisation current by an electroscope 

 orresponds to observation by eye or by the photo- 

 -^raphic plate. 



In the drawing of Fig. i,^ which shows the 

 arrangement of the apparatus in plan, Q is the 

 antikathode of the X-ray bulb. The construction 

 of the bulb is a little unusual in that the anti- 

 kathode is placed perpendicularly to the kathode 

 ray stream ; the bulb can therefore be conveniently 

 arranged so that the X-rays leave the antikathode 

 at a grazing angle. The finer the angle which 

 the rays make with the antikathode, the more 

 nearly does the source become, effectively, a 



bright" line; and the narrower the line the 

 brighter it becomes, because the "whole illumina- 

 tion " given out in any direction by the spot on 

 the antikathode is independent of the direction. 



' A summary of the principles on which this experimen' is baseJ, and of 

 the progress of its development may be found in Nature of July 9, 1914. 

 p. 4C<. 



2 Sc-e Proc. Camb. Phil. Soc, November it, 1912, or Proc. Roy. Soc., 

 April 7, ioi-<. 



' From a book now in the press, to be pu' lished by Messrs. O. Bell and 

 Sons. 



NO. 2347, VOL. 94] 



The law followed is not that of the illumination 

 by a surface of uniform luminosity, but rather 

 that of the illumination due to a number of 

 separate sources lying in one plane, each radiating 

 uniformly in all directions. It corresponds to the 

 case described by Rutherford, in which o rays are 

 radiated from a uniform thin sheet of radio-active 

 matter spread upon a plane surface (Phil. Mag., 

 August, 1906). The arrangement is of consider- 

 able value ; the more nearly is the source a bright 

 line parallel to the slit, the " purer " is the 

 spectrum. 



The X-ray bulb is enclosed in a wooden box 

 heavily coated with lead. The object is to protect 

 not merely the observer but also the sensitive 

 apparatus. The slit through which the rays pass 

 is only a few millimetres long and very narrow, 

 sometimes no more than a tenth of a millimetre 



Fig. I. — X-ray spectrometer. Plan of general arrangements. 



wide. Only a small fraction of the pencil that 

 emerges is reflected in the best of circum- 

 stances, so that it is necessary to screen off all 

 stray radiation with great care ; it must be small 

 in comparison with the radiation which is to be 

 measured. The side of the box is shown on the 

 left of the photograph of Fig. 2. Adjustable slits 

 are placed at A where the rays leave the box, and 

 again at B, the second slit being also capable of 

 a movement which brings it close up to the 

 crystal, as at B'. The crystal is shown at C, and 

 a third slit, D, is placed just in front of the ionisa- 

 tion chamber. 



The ionisation chamber is marked I in Fig. 2. 

 It is a cvlindrical brass chamber 15 cm. long, and 



