Bit different to my usual slides, but a very interesting one. This is what the label says – “Deep sea soundings. Cyclops 2330 fathoms or 2½ miles 260 yds [yards]. Lat[itude] 51.20 Long[itude] 38.0 [or 6 not sure] W[est].”. I added the bits in square brackets. This looks to be from the 1857 HMS Cyclops expedition surveyed the route for laying the first Transatlantic telegraph cable. Putting the coordinates into Google Maps gives the location as just to the west of the Mid Atlantic spreading ridge between Ireland and Newfoundland. No makers name for the slide. Mainly forams (Globigerina), but I did find a Gyrosigma diatom on there as well.
Olympus BHB microscope using 450nm LED light. The wider field of view was done with a 10x Nikon Plan Apo NA 0.45 objective. The Gyrosigma sp. with a 20x Nikon Plan Apo NA 0.65 objective. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. Single unstacked image. I couldn’t get close enough to use the 40x or 63x Leitz oil immersion objectives.
There may be more about this in the book: Deep Sea Soundings in the North Atlantic Ocean between Ireland and Newfoundland, made in HMS Cyclops, Lt and Cdr Joseph Dayman in June and July 1857, 1858.
EDIT – I did get a copy of the book mentioned above. On page 37 it gives the location Lat 51°29’N Long 38°0’W dated June 23rd 1857. This is the closest to the location details on the slide and was done on the outward journey. The bottom was quoted as 2250 fathoms, but does say 2300 in the chart. This is a little shallower than on the slide. Also from the page “The ship kept her place only tolerably well in this cast, and the intervals are not so regular as usual. The depth was however verified in the return voyage on the exact spot.”. Also note that in the abstract on Page 58, this location has the date of 13th July next to it. Maybe this was the return date when it was checked? There was also a depth of 2330 fathoms mentioned but that was for Lat 50°25’N Long 44°19’W dated June 26th 1857. Given the Lat and Long are not close to looking like what is written on the slide, I doubt it is this one.
So, note quite the cut and dried answer I was hoping for, however I suspect that the sounding on the June 23rd 1857 was the one this sample came from.