198 J' Stanley Rogers: 



FF', which has O as centre, and OS^ as radius, all the X-rays 

 •of a certain wave length X would be reflected to a particular 

 vertical line on the film. If there is present in the incident beam 

 X-rays of a definite wave length, carrying more energy than those 

 of adjacent longer and shorter wave legths, then its presence will 

 .be shown by a line on the film. 



An additional advantage of a rotating crystal is that it gives 

 much sharper lines than a stationary one, since the effect on the 

 lines of surface defects of the crystal is thus considerably les- 

 sened. 



No spectrometer being available, a Dancer theodolite was 

 modified to have the movements of a spectrometer, and to fulfil 

 the requirements of the focussing condition. It may be men- 

 tioned that a theodolite can be adapted to form a spectrometer, 

 which is both accurate and convenient. 



The circle carrying the scale was fixed by shrinking a brass 

 ring on to its under surface at AA' (fig. la). This ring was 

 .supported by a tripod mounted on the ring BB', through which 

 passed three levelling screws bearing on a stone table. The 

 crystal holder was mounted on the vernier circle at D. The 

 film holder was carried by an arm which screwed into the theodo- 

 lite at E. Both crystal holder and film holder turned on conical 

 bearings. The slits S^ and S^ were supported by a brass tube 

 which screwed into the ring BB'. 



The crystal was a large calcite one. The reflecting face 

 (5.cm. X 2. cm.) was a ground cleavage one. As W. L. Bragg, 

 James and Bosanquet^ have shown, ground faces are more effi- 

 cient reflectors than natural cleavage faces. 



The crystal holder was so designed that the reflecting face of 

 the crystal could be made vertical, and could be brought into the 

 .axis of rotation of the spectrometer. 



The film holder was cut from a brass ring, the inside edge of 

 which was accurately turned, its radius being 10.001 i. 001 -cm. 

 The film, in its paper casette, was tightly pressed against this 

 by means of two curved strips of red fibre and clamping screws. 



Slits. — The slit S^, the tube slit, was made of an alloy of lead 

 :and antimony (75 per cent, lead), such an alloy being more dur- 

 able than pure lead. The inside faces of this slit were carefully 

 ground, and the slit width could be adjusted by a screw to 0.005. 

 :mm. The slit Sg was made of lead, and was carried by the 



6. Fhil. Mag., XLI,, 1921, p. 309. 



