Microscopy – early Horace Dall aluminium coated diatom slide

A special slide to share today. I’ve shared a few images from slides made my Horace Dall before, and this is another one. Dall was an optics genius, and experimented with using titanium dioxide and aluminium layers to improve the visibility of diatoms. This slide is from 1960 and is marked up as being ‘Pleurosigma Balticum‘. Here are some images from it taken on my modified Olympus BHB microscope using 450nm light. All have been reduced in resolution for sharing here.

40x Olympus Dplan Apo UV objective, full image, stacked.
x 40 Olympus Dplan Apo UV image, stacked, cropped from full image
20x Olympus Splan objective, single image
10x Olympus UVFL objective

The aluminium coating has the effect of improving the contrast for the diatom, making the features more visible. These were imaged with simple bright field illumination from below. I’m not happy with my stack with the 40x objective, as there are artefacts in there, so I shall redo that at some point.

Here’s the slide itself.

The Dall Pleurosigma balticum aluminium coated slide made in 1960

The slide says it is ‘best seen with vertical illumination and with oil imm[ersion].’. However my system is not really setup for illumination from above, which is why I lit it from below. This slide appears in the Quekett Microscopical Club book, “Microscopical Mounts and Mounters” by Brian Bracegirdle (plate 13, D). It’s an early example for one of Dall’s aluminium coated slides, and looks different to the others I have which are shiny like mirrors. I suspect it is an early experiment using the technique.

EDIT – after a chat with an expert, while the slide is marked as Pleurosigma balticum, (new name, Gyrosigma balticum) it is not that, and is likely to be one of very big Pleuorsigmas like Pleurosigma formosum. This highlights an issue I have had with a few different diatom slides – that of naming. I’ve had a few slides where the name on it did not match the subjects.

As always, thanks for reading, and if you’d like to know more about my work I can be reached here.