Pilar smiled and shrugged her shoulders almost imperceptibly and said, "You know the way?"
It went slowly, almost imperceptibly, for it was a small watch and he could not see the second hand.
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.