ELECTRON MICROSCOPY 



w |. .,,1 m,..Jowincr be made, and two evaporations are required, 

 ' a high-resolution rephca can be obtained 

 When shadowing rephcas, various factors ^^.^^^^ platinum or uranium shadowed siUcon 

 must be taken into consideration. In the case j^^^^Q^i^g or carbon. If a fairly easily pre- 

 of plastic replicas it must be borne m mmd ^^^^^ replica of limited resolution is required 

 that the top surface of the replica does not ^^^^^ "Formvar" replicas from a metal sur- 

 accurately follow the surface topography of ^^^^ ^^^ suitable and can be made quickly, 

 the specimen beneath. If a plastic replica is to ^^ important factor is the strength of the 

 be shadowed the material must, therefore, be ^.^piip^^j^^g material. If the material tends to 

 deposited on the surface which has been in ^^^^^ ^^. ^^^^^^ -^^ ^^^ electron beam it will 

 contact with the specimen. It will then pro- ^^ difficult to obtain good electron micro- 

 duce a "negative picture" of the specimen ^^^^^is. Many workers have found that car- 

 surface, i.e., projections on the specimen will ^^^ .^ ^^^ strongest replicating material. It 

 appear as hollows on the repUca. This can ^^^ ^ further practical advantage in that it 

 be appreciated from Figure 10, which shows .^ ^^^^ ^^^.^^ ^.-g.^^g ^y transmitted Ught 

 the different ways in which a carbon replica ^^^^^^^ ^^^^^^^^ monoxide is difficult to see. 

 can be shadowed. 'pj^jg factor is of great importance in the 

 When carbon, siUcon monoxide and other j^^^^^^-^^g ^^-^^ mounting of replicas. A useful 

 evaporated replicas are to be used, shadow- ^^^.^^^^g^gg ^f carbon is that it can be treated 

 ing can be carried out in two ways. The speci- ^^ ^^^^ ^^^^^^^^ ^^-^^^ ^^^^i as hydrofluoric 

 men itself can be shadowed prior to the ^^.^^ without any effect on the film, 

 deposition of the replicating material, m if' stereoscopic electron micrographs are to 

 which case it is known as a "pre-shadowed ^^ obtained, it is essential that a shadowed 

 replica" (Figure 10a). Otherwise the replica ^^^^^^ ^^, ^.^.^^^^ monoxide replica be em- 

 can be shadowed on the outer surface after ^^^^^^ The contrast-forming mechanism of a 

 it has been mounted on the electron micro- ";pormvar" replica is such that it is notpossi- 

 scope grid (Figure 10b). In fact there is little ^^^ ^^ obtain satisfactory stereoscopic pairs, 

 to be gained from one or the other of these ^^^^,^ .^ ^^^ ^^^^^^ ^^^^ replicas and shad- 

 procedures, although a pre-shadowed replica ^^^.^^^ ^^^^ ^ ^^^^^ important part in electron 

 is obviously better laboratory practice, and j^^-p^-oscopy. Where they are not of prime 

 if a specimen is likely to be distorted during • ^^^portance in the study of a particular type 

 a replica process, it is essential. ^^ specimen, they can often be usefully 



employed in conjunction with such methods 

 Choice of Materials ^^ ultra-microtomy. 



The choice of a replicating or shadowing Acknowledgments. The author would like to 



material will of course depend on the nature thank Dr. T. E. Allibone, F.R.S., Director of the 



of the specimen to be examined. There are, Research Laboratory, for permission to publish 



however, a number of general points to be this article, 



considered. The first and most obvious is the references 



required resolving power of the ultimate ^ Schaefer, V. J. and Barker, D., J. Appl. 



electron micrograph. If a high resolution P/ij/s., 13, 427 (1942). 



image is required then a carbon or silicon 2. Agar, A. W. and Revell, R. S. M., Bnt. J. 



monoxide replica and Pt/C shadowing is ^ppL P%s., 7. 17 (1956) 



, . , , -^ , , 1 ■ 1 r +• 3 Keller, F. and Geisler, A. H., Trans. Am. 



desirable. Both shadowmg and replication ^^^^ ^.^ ^^^ ^^^^ ^^^^ g, (1944). 



can be carried out m one single evaporation ^ Welsh, N. C, J. Inst. Metals, 85, 129 (1956). 

 by using "self-shadowed" platinum/carbon 5 Hunger, J. and Seeliger, R., Metallfor- 

 replicas. Where a pre-shadowed replica is to schung, 2, 65 (1947). 



238 



