How often do you actually see glass? Most of the time I don’t see the glass, I see whats on the other side, because modern glass – be it windows, camera lenses, spectacles – is clear an almost invisible. For specs and lenses they are even coated to avoid unwanted reflections in order to create the most undisturbed view of whats on the other side. Of course when you glass is dirty you’ll notice, but not the glass, the dirt.
It has not always been this way, going back in time to early phases of glass usage in architecture you’ll find glass that you can avoid seeing. In the late 15 hundred or early 16 hundred years glass production was far from perfect, windows would alter what you could see. Alterations could both be color changes and disproportions to what you see. I have just visited the famous castle Kronborg – the one where Hamlet takes place – which originates from 1585 and window glass there is definitely not perfect, just have a look at the pictures.