In article <4tc6ua$pm5@news.nstn.ca>, chefurka@fox.nstn.ca says...
>In article <4t5p9v$dh9@newsstand.cit.cornell.edu>, d_ruether@hotmail.com (Bob >Neuman) says:
>>The "glass" (actually, most of the screens have a glass condenser lens
>>on top, with a plastic GG and Fresnel lens on the bottom, with a thin
>>dust-leaky tape holding them together) is the same size in the screens
>>of the F, F2, F3, and F4, and all interchange in the two different
>>frame types (F/F2 - F3/F4).

>Hmmm. I just tried it, and *it doesn't work*. The F3 "glass" assembly
>is about 1/2 mm. too big in both dimensions to fit the frame from the F.
>I now suspect that F/F2 and F3/F4 screens actually are different
>"families".
>So close, and yet so far...

You are right! (Memory failed me - I remembered interchanging them....)
I just checked, and F3 "glass" is slightly longer and noticeably wider
than F2 glass - they do not interchange!
Uh, Hope This Helps...... ;-)