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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
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Regardless of whether the dislocations are of edge- or screw-type, based on the invisibility criterion, the dislocations are invisible at g·b = 0 or exhibit a so-called "residual" contrast for edge dislocations when the dislocations are out of contrast [1], the dislocations are visible in the other cases.
Similar to edge dislocations, the character of pure screw dislocations is also independent of the direction of the sense vector. As shown in Figure 3458, in the system of right-handed screw dislocations, regardless of the sense vector, the sense vectors and Burgers vectors are in parallel direction and thus,
 ------------------------------ [3458a]
On the other hand, in the system of left-handed screw dislocations, the sense vectors and Burgers vectors are in opposite directions and thus,
 ---------------------------- [3458b]
Figure 3458. Sense vector and Burgers vector for a right-handed screw dislocations.
Refer to page3460 for the notations. Practically, only one group of expressions is used, for instance, using ξ and b in stead of ξ1 and b1, and ξ2 and b2 simultaneously. Furthermore, a glide dislocation is generally a line boundary between the slipped and unslipped portions of the glide (slip) plane. The line is not necessary to be straight. When it is curved some parts can be a screw dislocation and some parts an edge dislocation. For a given sense vector that points in one direction along the dislocation, the Burgers vector is invariant.
[1] Hirsch, P., Howie, A, Nicholson, R.B., Pashley, D.W. and Whelan, M.J., 1977,
Electron Microscopy of Thin Crystals, Krieger, New York, p. 181.
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