Imperceptible
Imperceptible , adjective
[Prefix im- not + perceptible: compare French imperceptible.]
Not perceptible; not to be apprehended or cognized by the senses; not discernible by the mind; not easily apprehended.
Almost imperceptible to the touch.
Its operation is slow, and in some cases almost imperceptible.
Their... subtilty and imperceptibleness.