Just for clarification. The condenser sets consist of 2 concave lenses in the silver metal tube; concave surfaces facing each other. You need both of the lenses to focus the light. The condenser size is the diameter to the lenses. The metal tube they are mounted in is 6 1/2" in diameter, so the 6 1/2" condenser set fills it up. Measure the diameter of the condenser lens, that will tell you what you have.

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