Create an object that tapers so thin that its thinnest portion cannot be seen by the eye. Perhaps fiber optics. It might be able to be sensed by touch, brushing against the skin. No known practical purpose: just the cognitive dissonance of an object that seems simultaneously there and not there.
Inspired by the thin end of hair.
So thin as to be invisible is easily achieved by decreasing vision acuity, though that seems like cheating.