While the ear of the former has an external opening, that of the latter is entirely and evenly covered over with a membrane, so as to be quite imperceptible from without.
Moreover, while in most other animals that I can now think of, the eyes are so planted as imperceptibly to blend their visual power, so as to produce one picture and not two to the brain; the peculiar position of the whale’s eyes, effectually divided as they are by many cubic feet of solid head, which towers between them like a great mountain separating two lakes in valleys; this, of course, must wholly separate the impressions which each independent organ imparts.
There are no more uses of "imperceptible" in the book.
Show samples from other sources
...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.