imperceptible

Miss Mortimer gave a nearly imperceptible sigh of exasperation, but still she waited.

But the opposite, reflective face bears an image incised in finely engraved lines that under normal conditions is virtually imperceptible to the naked eye.

"Traditionally, the intense heat of firing destroys any biological residue. However, our technique has been able to reveal fingerprint ridges that would otherwise remain imperceptible."

The hum of the electric motor is imperceptible, and it is only at speed that the sound of the rolling wheels on the road breaks through.

Five years later, Mini sat on Khorram’s lap, her disability nearly imperceptible — and in any case eclipsed by her purple-dyed tail.