If about the material under the cover glass, there was nothing special except the sample - drop of water with some green algae. I noticed that not every objective gives such good results. It of course depends on their depth of field and apertures. I was trying this frame with 20 X Splan Apo and the result was disappointing.
The extremally shallow depth of field has its good points. As an example of it I am enclosing additional image presenting very similar place ( or the same, I am not sure of it) of the cover slip. Thanks to shallow depth of field I could obtain quite different result.
The equipment was the same like in the previous case.