INVISIBLE: unable to be seen; not visible to the eye.
In centuries past, things like bacteria, the amount of 'life' in a drop of water, and yeast plants were 'invisible.' No one could see them or even knew of their existence. It wasn't until the late 17th century that a man invented a microscope that allowed us 'to see.' Apart from the microscope, these things are invisible to our eyes. We know when we're affected by bacteria, but we can't see it being passed along to us. We see water as transparent, not seeing all of the 'minerals added for taste.'