Epithemia turgida, monobromonaphthalene, Watson and Sons

Epithemia turgida
Epithemia turgida strew, 10x Nikon Plan Apo NA 0.45 objective
Epithemia turgida strew, 10x Nikon Plan Apo NA 0.45 objective

A strew slide of Epithemia turgida (with examples of other species also present). Also known as Navicula turgida. Prepared by Watson and Sons Ltd. Mounted in mono-bromide of napthalin (monobromonaphthalene). A lot of the mountant has been lost, and very little can still be imaged. The mountant around the edge of the coverslip has gone brown (perhaps it has solublized something from the ring around the coverslip). Olympus BHB microscope using 450nm LED light. 63x Leitz Pl Apo NA 1.40 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, brightfield lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 17 images stacked in Zerene (Pmax).

Monobromonaphthalene was used as a high refractive index mountant however it seems to have fallen out of favour relatively quickly and it is now rare to find a slide with it. It is reported to have a refractive index of 1.656-1.659 so is certainly high with respect to materials such a Canada Balsam (about 1.54) and Styrax (about 1.58), but with the advent of higher refractive index mountants such as Hyrax, Naphrax and Pleurax there would have been less of a need to use it. The reduction in the size of the regions of the mountant seems seems to have taken the diatoms with it as it has moved.

Slide donated by Nigel & Lisa Ashby.