= Ultramicrotome
An <ultramicrotome> is a precision cutting instrument for producing very thin sections or successively exposing new specimen faces for <electron microscopy>. <Ultramicrotomy> combines a controlled specimen feed with a cutting stroke across a sharp <diamond knife> or suitable glass knife. A polymer block is one example of a specimen for materials work. JEOL describes preset feeds of tens of nanometres and notes that actual section thickness can differ from the setting.<Ultramicrotomy by JEOL>[Definition, specimen advance and diamond-knife cutting description; thickness dependence on material and temperature]{ref}
The mechanism must distinguish the feed direction, which sets the intended removed thickness, from the much longer cutting stroke across the block. Section collection supports <transmission electron microscopy>; an in-chamber version supports <serial block-face scanning electron microscopy> by imaging the remaining surface.
For an original cutting-budget example, suppose a polymer block is advanced by $60\,\mathrm{nm}$ per cut. After 250 uniform cuts the nominal removed thickness is
$$
L=250\cdot60\,\mathrm{nm}=15.0\,\mathrm{\mu m}.
$$
If one entire section is lost during collection, its neighbors' centers are $120\,\mathrm{nm}$ apart rather than $60\,\mathrm{nm}$, assuming all feeds were correct. The total removed thickness remains $15.0\,\mathrm{\mu m}$; the lost section leaves an observation gap. Recording only the number of surviving images would conceal that gap.
A physical check compares the intended feed with an independent thickness or accumulated-depth measurement, while recording specimen and knife conditions. A fine actuator increment alone does not establish the actual section thickness. Compression, uneven cutting and missing sections affect the geometry subsequently assigned to an <image stack>.
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