discernible

And it led on and on, utterly undiscernible to the Major or Falkner, hardly visible to Kendrew at times, but plain enough to me.

Thus, for some undiscernible reason, duels are not infrequent, which generally end in the death of one or both combatants.

Undiscernible, un-di-zer′ni-bl, adj. indiscernible—also Undiscer′nable.

It seemed incredible that mere paths, often all but undiscernible close at hand, should be distinguishable from this distance.