A strew slide of Cyclotella rectangula (and other species) from Paris. Prepared by Theodor Ernst Eulenstein. More on Eulenstein here. Interestingly the slide says ‘α-bromonaphthalene, 2-5-51’ which is handwritten at the bottom of the label with the species name on it. Is this a mono bromonaphthalene mount, and if so, why is it only mentioned in a hand written note from what is presumably 1951, when the slide was made in the 1860s? Very mysterious.
Olympus BHB microscope using 450nm LED light. I picked one of the larger examples I could find (C. rectangula is a very small diatom) for further imaging. Firstly, 100x Leitz Pl Apo NA 1.32-0.60 objective, oil immersion, set to wide open NA 1.32. Olympus Aplanat Achromat condenser, oil immersion, very oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 13 images stacked in Zerene (Pmax). One central fultoportulae visible. Some of the details were lost in the stacking. and looking at a single frame and boosting the contrast a bit shows a hint of the chambers in the striae. I had hope to see something like the wonderful transmission electron microscope images of Rex Lowe which show the interior structure of Cyclotella atomus, shown here. However this was ‘overly ambitious’ for optical microscopy. Measurement in ImageJ showed the dot spacing to be about 312nm.
I also tried dark ground imaging. Same objective (but with the objective iris closed down until I was able to get darkground) and general setup, Reichert Neo oil immersion darkground condenser, single frame. However even with this I couldn’t get much more details. Measurement of the ‘dot’ feature again came out as 306nm, so similar to the oblique image.
This name is currently regarded as a synonym of Cyclotella meneghiniana var. rectangula (Kützing) Grunow. Some information on that (along with some images) in Kaoli Pereira Cavalcante, Priscila Izabel Tremarin, Thelma Alvim Veiga Ludwig. Taxonomic studies of centric diatoms (Diatomeae): unusual nanoplanktonic forms and new records for Brazil. Acta Bot. Bras. 27 (2) June 2013. https://doi.org/10.1590/S0102-33062013000200001. Also, Cyclotella meneghiniana is a synonym of Stephanocyclus meneghinianus (described here).
Overall a very small species and it pushed my resolution abilities to see and image. I did try 365nm light, with the Reichert Neo condenser, but I got virtually no light through the slide. This makes me wonder if this is indeed mono bromonaphthalene mount, as if it was Styrax of Canada Balsam, I would have expected to get some UV through it. I’ll measure the transmission through it when I next have the spectrometer out.