Let me be clear... a bit about glass

I found a photo I took of a rainbow last summer when we drove across Canada. I took it from a moving car (I wasn't driving) - so it isn't as fantastic as it could be.

I've been thinking about Theodoric of Freiberg's rainbow experiment (I wrote initially about it here). He used a spherical glass filled with water to approximate a rain drop and a piece of parchment with a pin hole in it. By shining light through the parchment hole and onto the glass sphere, he was able to observe the result of raising and lowering the sphere. From this he explained all the colours of a rainbow. So his glass sphere must have been essentially perfect for this to work, and he wasn't the only one using these sphere's for optics experiments. So how did we get so good at making glass? (the extremely short version)

Glass making is an old art, by about 1500 BC the Egyptians were making glass vessels and soon after the Phoenicians mastered the art and began exporting glass goods all over. However, the Romans with glass blowing (likely invented by the Phoenicians), put cheap glass vessels into their citizen's homes. Romans went on to adapt glassblowing for making glass windows for some of their buildings – not widely done because they lived in a warm environment. Roman windows were made by blowing glass into a bulb shape, then manipulating it into a cylinder shape. The cylinder was split open lengthwise before being re-heated and forced flat. One of the largest windows made of this method was found in Pompeii measuring just over a metre wide.

So, the Romans weren't hugely into glass windows, but after they were gone, those who lived to the north took up interest in them. The technique used became simplified to blowing glass into spheres and then cutting them while still hot into the shallow bowls of 'crown-glass' windows. Additives of different minerals result in brilliant colours for stain glass windows. Since, churches were one of the few places rich enough to afford glass windows and they wanted to tell stories through cut coloured glass put back together, large sheets of uniform glass wasn't necessary.

Throughout medieval times, rich folks were drinking out of blown glass and keeping the weather at bay with blown glass windows. So by, Theodoric's time in the fourteenth century, glassblowing had been around a long time. If an artisan can make a nice wine glass, certainly that skill could be put to use for scientific instruments.