Whenever he smiled no obvious movement was required of his facial muscles; the hint of mockery was always there and it merely came into sharper focus for a moment, to recede imperceptibly again.
He had known for several years the trend which his paper had embraced gradually, imperceptibly, without any directive from him.
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.