Nature Publishing Group 1962 'lo.4ats March 3, 1962 NATURE 867 from 1961. and the tendency seems to be the same. Analysis of single samples during the spring of 1960 indicates that the French low-yield tests of February and March that year produced no significant effect on the monthly fall-out of loug-lived radioactivity in Sweden.Owing to the low activity-level of the fall-out from weapons tests during 1960 and up to September 1961 it has been possible to carry out a quantitative examination of the presence of beryllium-7 {produced by cosmic rays) in ground-level air {Fig. 2). Beryllium-7 is a y-emitter with its main photopeak at 0•48 MeV. and 53 days half-life. There seems to be considerable variation with time. There are, however, some difficulties involved in the analysis of the spectrum', and the result is only preliminary.

W. G. BURGERS 503 stration apparatus to an irradiation of the model by the 'plane of light' D (produced by the lamp A inside the box B and the vertical slit C). The other orientations of the crystallites perpendicular to the incident beam can be obtained starting from this arbitrarily chosen orientation by (a) a rotation about the c* axis, (b) a rotation of the whole set of orientations so obtained about an axis perpendicular to the c* axis, i.e. parallel to the direction of the incident electron beam.The first rotation leaves the' disk' (0002) in its original position and moves the other disks outside the ' plane of reflexion'; it is therefore immaterial and need not be realized.'[" The second rotation can be achieved by means of the motor M, which causes the model to rotate about the axis N, parallel to the electron beam. If now we use for lamp A a sodium or mercury gas-discharge lamp J: we obtain the stroboscopic effect shown in Fig. 2, which is a photograph of the actual pattern visible in a darkened room. The demonstration illustrates in a rather striking way the difference in direction of the individual diffraction striae, namely, perpendicular, parallel and oblique to the diffraction circles, as it has to be on the basis of the actual shape transform in reciprocal space.The resemblance with the electron-diffraction photograph, however, cannot be expected to be more than 'qualitative', as diffraction by needles in an oblique position with regard to the incident beam, although not t This rotation would, however, bring the (11~0) domain, discussed in Rees & Spink's paper, into the 'plane of reflexion' and show the 'radial' direction of the corresponding interference striae. As this would require an extension of the model in the direction of the b* axis, which would make it more 'voluminous', we have left this out, as the same effect could be shown more easily for the (10i0) reflexion.J: I am indebted to Dr F. J. Lebbink for this suggestion.giving rise to (0002) reflexions on account of the small value of the glancing angle 0, might give reflexions such as (1010), for example. These reflexions would show shapes corresponding to a different 'cross-section' of the 'plane of reflexion' with the disk-shaped shape transforms. Actually, the (lOiO) diffraction ring shows, apart from striae perpendicular to the circumference of the ring, also more 'roundish' (disk-shaped) spots.

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