Microscope Coverslip — The Thin Glass That Flattens Specimens
Microscope Coverslip — The Thin Glass That Flattens Specimens
Standardised cover glass for high-magnification gem microscopy
A microscope coverslip is a thin glass cover, typically 0.13 to 0.17 mm thick, placed over specimens on a microscope slide to protect the sample, flatten it for examination, and provide the optical thickness for which short-working-distance objectives are corrected. In gemmology, coverslips are most often encountered when mounting inclusions extracted from a stone or when examining surface features in immersion fluid.
Standards
The standard coverslip square is 18 × 18 mm or 22 × 22 mm; rectangular formats up to 24 × 60 mm are also stocked. Thickness is graded as #0 (0.085 to 0.13 mm), #1 (0.13 to 0.16 mm), #1.5 (0.16 to 0.19 mm), and #2 (0.19 to 0.23 mm). High-magnification objectives are typically corrected for #1.5 thickness; using the wrong grade introduces spherical aberration and degrades image quality. Most gem-microscopy work uses #1 or #1.5.
Use in gemmology
Coverslips are essential when an inclusion or surface fragment has been extracted from a stone for high-resolution examination. The fragment is mounted in immersion oil or mounting medium between slide and coverslip, allowing objectives in the 40× to 100× range to focus through the cover and resolve fine detail. They are also used in immersion-cell work, where the coverslip seals the immersion fluid against the gemmological microscope objective and provides a flat optical interface.
Handling
Coverslips are fragile and easily contaminated. They should be handled by the edges with clean tweezers, stored in their original holders, and discarded after use rather than cleaned and reused, as residual contamination from previous specimens can compromise photomicrography and fluorescence work.