Im′per‐cep″ti‐ble (?), a. [Pref. im- not + perceptible: cf. F. 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. Dryden.
Its operation is slow, and in some cases almost imperceptible. Burke.
— Im′per‐cep″ti‐ble‐ness, n. — Im′per‐cep″ti‐bly, adv.
Their... subtilty and imperceptibleness. Sir M. Hale.