Slide labelled as Campylodiscus kittonianus (more on this in a minute). Two diatoms on the slide. No location given, but the label does say ‘very rare’. No makers name, but the style and hand writing is that of Arthur Charles Cole. Olympus BHB microscope using 450nm LED light. I couldn’t get close enough with the 40x or 63x Leitz Pl Apo objectives I normally prefer to use, the two were imaged with 10x and 20x Nikon Plan Apo’s an oblique lighting (with the 20x image the space between them was reduced to bring them closer together).
As the slide was relatively low contrast (I wouldn’t be surprised if this was Canada Balsam), I did a bit of an experiment, and used a 50x Leitz Fluoreszenz NA 1.00 objective, water immersion. Lomo oblique condenser, water immersion. 365nm light (to help with resolution and contrast). 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 78 images stacked in Zerene (Pmax). The long working distance of the objective, and the use of 365nm light both worked well here and allowed me to get a relatively high resolution image.
Back to the name. Looking through Deby, J., 1891, Analysis of the Diatomaceous Genus Campylodiscus. Privately published, London, I think both of these are probably Campylodiscus horologium, Plate VI, Figure 29a. However I will leave a question mark after that for now.