https://kwtelescope.com/double-concave-microscope-slides-72-slides-per-box/
Walter Products Double Concave Microscope Slides 72 slides per box
KW Telescope-Perceptor BigCommerce Store
walter productsdouble concavemicroscope slidesperbox
https://www.flinnsci.com/lens-double-concave-38-mm-dia.-20-cm-fl/ap5679/
Double Concave Lens with 20 cm Focal Length
Lens, Double Concave, 38 mm dia., 20 cm FL, Round student lenses are top-quality flint glass lenses and useful in a variety of optical experiments.
double concavelenscmfocallength