Amician Test slide, Watson and Sons

Navicula rhomboides, Amician Test

Amician Test slide with Navicula rhomboides (now known as Frustulia rhomboides). No location given. Made by Watson and Sons. Says “Test Dry covered for 1/25th Inch”. Olympus BHB microscope using 365nm LED light. 100x Leitz Pl Apo NA 1.32-0.60 objective, oil immersion, iris fully open. Reichert Neo 1.42/1.18 dark ground condenser, oiled. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. No stacking.

Puncta spacing 373nm. Striae spacing 344nm. I had expected this to imaged in circular oblique illumination with the iris on the objective fully open, but it never changed from dark ground. I suspect this is due to the dry mount.