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.
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         Almost imperceptible to the touch.       --Dryden.
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         Its operation is slow, and in some cases almost
         imperceptible.                           --Burke.
   -- Im`per*cep"ti*ble*ness, n. -- Im`per*cep"ti*bly, adv.
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         Their . . . subtilty and imperceptibleness. --Sir M.
                                                  Hale.
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