You can argue that this was "imperceptible" rather than "invisible". But you could say the same thing about things we only see with the aid of telescopes and mi

You can argue that this was "imperceptible" rather than "invisible". But you could say the same thing about things we only see with the aid of telescopes and microscopes. "The amoebas weren't invisible; that light was already bouncing up to your eyes." But really, invisible means "unable to be seen." Unless you can demonstrate that it's possible for a person to see these things unaided, they are invisible.