A strew slide mixed test diatoms, including Pleurosigma, Gyrosigma, Surirella and others. Material provided by Bernard Hartley. Slide was prepared by Horace Dall. Shadowed with aluminium in a vacuum. Standard brightfield illumination from below gives a darkground image with full NA even for a high NA objective. Olympus BHB microscope 450nm LED light. As well as the wider field of view image, I photographed a Pleurosigma sp. (probably P. angulatum). 63x Leitz Pl Apo 1.4, oil immersion. Olympus Abbe condenser, oil immersion, brightfield lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. Single unstacked image. Quite a thick aluminium layer on this one (some are thicker than others).
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, Figures 7, 8 [as var. beta], 9 [as var. gamma].