a letter written in 'invisible ink' [=ink that can only be seen when it is specially treated in some way]
It felt like I was 'invisible' at the party. [=no one was paying any attention to me at the party]
With the telescope we can see details of the planet's surface that are ordinarily 'invisible'.
(..) Where the hell did my new mousse go? We're gonna have so much fun, baby girl. The skies aren't smoggy there, so you get to see real stars. Not the fuzzy invisible LA stars. And you get to play with Spencer, smell your first campfire. Shall we bring your baby Deadhead onesie, or is that too obvious? (..)2003 TvShow: Six Feet Under Title: Making Love Work Season: 3 Episode 6