A strew slide of Pleurosigma angulatum (and others). Mounted in Pleurax (refractive index given as 1.90, which I think is a tad optimistic) and dated July 1934. Prepared by Samuel Henry Meakin. Very clear, high contrast mountant. Olympus BHB microscope using 450nm LED light. I chose an example of Pleurosigma angulatum for imaging at higher resolution. 63x Leitz Pl Apo 1.4 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 23 images stacked in Zerene (Pmax). I struggled a bit with choice of diatoms for imaging with the 63x objective with this one as the coverslip ring was quite high. Hence the broken frustule of P. angulatum.
Pleurosigma angulatum (J.T.Quekett) W.Smith 1852. Published in: Smith, W. (1852). Notes on the Diatomaceae with descriptions of British species included in the genus Pleurosigma. Annals and Magazine of Natural History, Series 2 9: 1-12, 2 pls. Page 7, Plate 1, Fig. 7, 8 [as var. beta], 9 [as var. gamma].