Translations for imperceptible in French

Here are paragraphs from public domain books translated:

A second smile, almost imperceptible, stole over the rosy lips of the pretty young woman.	Un second sourire presque imperceptible effleura les lèvres rosées de la belle jeune femme.
The Three Musketeers, by Alexandre Dumas	Les Trois Mousquetaires, de Alexandre Dumas