Blackadder. That would be because the heat goes to the back of the instruments heating the humid air inside. The glass, which is in contact with the outside air is getting very little of the warm air flow and remains cold by comparison to the air inside the instruments. The moisture in the air then condenses on the inside of the glass. Once the glass warms to the temperature of the air inside the instrument the misting will start to evaporate again.

Peter

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