But again, amidst the incontrovertible facts just recorded, one more, equally significant, rose up to confront the family; and this was, that the eldest daughter, Alexandra, had imperceptibly arrived at her twenty-fifth birthday.
Imperceptibly the conversation grew more animated and more interesting, so that neither of the two felt anxious to bring it to a close.
There are no more uses of "imperceptible" in the book.
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...and he let the line slip down, down, down, unrolling off the first of the two reserve coils. As it went down, slipping lightly through the old man’s fingers, he still could feel the great weight, though the pressure of his thumb and finger were almost imperceptible.