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