Interesting the comments about the bubbles being detrimental to the optics. For most of last century, having bubbles in the lens elements was actually a sign of

Interesting the comments about the bubbles being detrimental to the optics. For most of last century, having bubbles in the lens elements was actually a sign of a great lens! It was eventually narrowed down on how the glass was cooled (confirmed by this article) and it seems the slight change of refraction was largely compensating for the bubble(s). I've got some early, pre first world war lens that are absolutely amazing optically, bubbles and all. I've also got one Tessar 4 element lens in particular, on a pre 1936 (uncoated) Zeiss 6x9 Folder that is just out of this world in sharpness.