Source : Webster's Revised Unabridged Dictionary (1913)

Imperceptible \Im`per*cep"ti*ble\, a. [Pref. im- not +
   perceptible: cf. F. imperceptible.]
   Not perceptible; not to be apprehended or cognized by the
   souses; 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 . . . subility and imperceptibleness. --Sir M.
                                                  Hale.

Source : WordNet®

imperceptibly
     adv : in an imperceptible manner or to an imperceptible degree;
           "the power of the Secretary of State in London
           increased gradually but imperceptibly" [syn: {unnoticeably},
            {observably}] [ant: {perceptibly}]