And, for the first time, some of the bright glow which had enveloped him since the first day she fell in love with him began to fade imperceptibly.
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.