The Collaborative International Dictionary of English v.0.48:

Imperceptible \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.
   [1913 Webster]

         Almost imperceptible to the touch.       --Dryden.
   [1913 Webster]

         Its operation is slow, and in some cases almost
         imperceptible.                           --Burke.
   -- Im`per*cep"ti*ble*ness, n. -- Im`per*cep"ti*bly, adv.
   [1913 Webster]

         Their . . . subtilty and imperceptibleness. --Sir M.
                                                  Hale.
   [1913 Webster]

WordNet (r) 3.0 (2006):

imperceptibly
    adv 1: in an imperceptible manner or to an imperceptible degree;
           "the power of the Secretary of State in London increased
           gradually but imperceptibly" [syn: imperceptibly,
           unnoticeably] [ant: noticeably, observably,
           perceptibly]