He heard an almost imperceptible chuckle.

It is, however, probable that the one class runs into the other by imperceptible gradations.

He found that the electricity of the tourmaline decreased rapidly from the summits or poles towards the middle of the crystal, where it was imperceptible; and he discovered that if a tourmaline is broken into any number of fragments, each fragment, when excited, has two opposite poles.

The emotions were almost imperceptible, and it had taken her a long time of studying him to read him.

For example, from the evidence of molar changes due to the obvious parts of bodies, science first comes to believe in molecular changes due to imperceptible particles, and then tries to conceive the ideas of particles, molecules, atoms, electrons.