He's explaining that something can be invisible either by being imperceptible to our eyes, or by being hidden from our eyes. Invisible as in "not visible", "incapable of being seen"; whether because it's transparent or imperceptible to us or hidden (like that iron piece in the bottom of the ocean), it's all not visible. Unseen might work better in some cases. The problem lies in the different level of understanding of the world by him and his contemporaries as opposed to today's people -it boils down to physics, in other words, I think.
In modern Greek the meaning of the word is stricter and is the equivalent of the English "invisible".