Canada Balsam is what has been the commercial/scientific standard for gluing
optics together for a long time. One of the things that is nice about it is
that it is dissovable so repairs can be made. More modern adhesives (UV
curable adhesives) are a lot more difficult in the regards to repairability
and are not as recommended for large optics due to the lack of flex when in
the joint to be glued together.

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Why isn't there an Ozone Hole at the NORTH Pole?