Mystifying Orb Rings

Photo date: November 8, 2017. Photographer: Mark Mahin.

I took the photos below while photographing nothing but ordinary drops of pure, clean water falling against a dark background.  We see the astonishing phenomenon I call orb rings.

Are what we seeing holes in the orbs, or just dark marks? A closeup of part of the photo above clearly shows holes we can look right through.

The same thing is shown in the photo below from the same day.   Notice how we see here two holes in the orbs, one large hole at the top, and one small hole at the bottom.

The appearance of these orb rings on this day was astonishing. I repeated the same photographic setup on several subsequent days, with the same camera and the same procedure. No such rings appeared.