difficult to see

See: impalpable
References in classic literature ?
Sometimes he almost abandoned it for he found himself time and again wondering how slight might be the provocation necessary to trick him into making a bona-fide offer of marriage to Meriem if he permitted himself to fall more deeply in love with her, and it was difficult to see her daily and not love her.
Critics say the galaxies the Tyson team is seeing may just be relatively nearby dwarf galaxies, small galaxies that are difficult to see outside of the "local group" that surrounds our own Milky Way.