SELECTED DIFFRACTION 



ning instrument concerns the observation of 

 variations of potential across the surface of 

 a specimen (17). This may be done by suit- 

 ably arranging the conditions at the input 

 aperture of the collector. A study of reverse- 

 biased p-n junctions has been carried out 

 using this facility (17). 



Other applications have included a study 

 of the forming processes in point -contact 

 rectifiers (7) and the direct examination of 

 the small bores of spinnerets (8). 



The optical system of the scanning micro- 

 scope is basically similar to that of the x-ray 

 projection and x-ra}^ scanning microscopes. 

 It is possible, therefore, that an instrument 

 combining all three facilities could be con- 

 structed using special adapters to convert 

 from one mode of operation to another. 



A versatile scanning instrument (25) 

 which has facilities for x-ray projection 

 microscopy and which may also be used for 

 conventional transmission microscopy has 

 been designed and constructed in the Engi- 

 neering Department, University of Cam- 

 bridge. 



REFERENCES 



1. Knoll, M., Z. Techn. Phys., 16, 767 (1935). 



2. VON Ardenne, M., Z. Phijs., 109, 553 (1938). 



3. VON Ardexne, M., Z. Techn. Phys., 19, 407 



(1938). 



4. ZWORYKIN, V. K., HiLLIER, J., AND SnYDER, 



R. L., Bull. Atn. Soc. Test. Mater. 117, 15 

 (1942). 



5. ZwoRYKiN, V. K. et al., "Electronic Optics 



and the Electron Microscope" (Wiley, 1945). 



6. McMuLLAN, D., Proc. Inst. Elec. Engrs., Pt. 



II, 100,245 (1953). 



7. Smith, K. C. A. Ph.D. Dissertation, Cam- 



bridge, 1956. 



8. Wells, O. C. Ph.D. Dissertation, Cambridge, 



1957. 



9. EvERHART, T. E. Ph.D. Dissertation, Cam- 



bridge, 1958. 



10. LiEBMAN, G., Proc. Phijs. Soc, B, 68, 682 (1955). 



11. CossLETT, V. E., "Practical Electron Micros- 



copy" (Butterworths, London, 1951). 



12. Langmuir, D. B., Proc. hist. Radio Engrs. 25, 



977 (1937). 



13. Haine, M. E. and Einstein, P. A., Brit. J. 



Appl. Phys., 3, 40 (1952). 



14. Jacob, L., Phil. Mag., 28, 81 (1939). 



15. Moss, H. Brit. Inst. Radio Engrs., Pt. I, 5, 



10 (1945). 



16. Rose, A., "Advances in Electronics," 1 (Aca- 



demic Press, N. Y., 1948). 



17. EvERHART, T. E., Wells, O. C, and Oatley, 



C. W., (to be published). 



18. Bloomer, R. X., Brit. J. Appl Phys., 8, 83 



(1957). 



19. WoRONCOW, A., J. Inst. Elect. Engrs., Pt. Ill 



A, 93, p. 1564 (1946). 



20. Lax, L. and Weighton, D., Proc. Inst. Elect. 



Engrs., Pt. Ill A, 99, 804, (1952). 



21. Smith, K. C. A. and Oatley, C. W., Brit. J. 



Appl. Phys., 6, 391 (1955). 



22. McAuslan, J. H. L. and Smith, K. C. A., 



"Proc. Stockholm Conf. on Electron Micros- 

 copy" (Academic Press, New York, 1956). 



23. Fert, C, "Conf. on Recent Progress in Cor- 



puscular Microscopic Techniques," Tou- 

 lou.se, 1955. 



24. Page, D. H., Brit. J. Appl. Phys. ,9, 60 (1958). 



25. Smith, K. C. A., (to be published). 



26. Bernard, A., Bryson-Haines, D., Mulvey, 



T., J. Sci. Instruments, 10, 438 (1959). 



27. Thornley, R. F. M., European Regional Con- 



ference on Electron Microscopy, Delft, 1960. 



K. C. A. Smith 



SELECTED DIFFRACTION 



Selected diffraction electron microscopy is 

 that type of microscopy in which one or 

 more of the diffracted beams from the object 

 ("Bragg reflections") is selected by means 

 of a suitable aperture system and used to 

 form the image. All the transmitted rays 

 and the remaining diffracted rays are 

 stopped out and prevented from contributing 

 to the formation of the image. In this way it 

 is possible to observe alone that part of the 

 image responsible for a particular part of 

 the diffraction pattern. It may be regarded 

 as the converse of selected area electron 

 diffraction in which a small part of the ob- 

 ject is selected and the diffraction pattern of 

 that part alone obtained. 



Two methods of achieving selected diffrac- 

 tion images are showTi in Figure 1. The first 

 method may be achieved with any electron 

 microscope having an externally adjustable 



251 



