(Science: microscopy) thin glass, plastic, sapphire, SiO, NaCl, etc. Shaped into circles, squares, or rectangles for covering the specimen. Glass coverslips, grade no. 1 1/2, contain a high proportion of covers from 0.17 to 0.18 mm in thickness, close to the lens designer's specifications for employment of achromatic and apochromatic objectives of high numerical aperture. The thickness and refractive index of the glass coverslip are important because certain of their values have been assumed by the designer and manufacturer. These values have been specified by ASTM: thickness, commercial number 1 1/2: 0.16 to 0.19 mm, refractive index measured at the sodium D lines (589.3 nm), 1.523(0.005).