I seem to remember that in my early days I used a Gnome and this had 3.5 inc condensers and in the 'tube' there was also a seperator to keep the condensers apart.
This was a simple flangeless tube that fitted snugly and held the glass at a pre-set distance apart so to get optimum light spread and even-ness.
The air gap was something like 1/4 inch
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