Viewfinder Glass

Here's one for you guys:

If a Graflex viewing screen consists of one piece of ground glass then why does a Graflok back have two pieces of glass, a ground glass then, what appears to be, a Fresnal lens? Do you need both? The Fresnal lens alone is very bright but it has an extra bright circle within the viewing frame when viewed without the diffuser.

not sure, but when I put a fresnel lens into my rollei on top of the ground glass it gets a lot brighter. I suspect the fresnel acts as a lens (which is what it is, essentially) while the frosted glass is still the flat surface on which the image is projected.

The fresnel focuses (columnates actually) the image a bit making the ground glass (GG) appear brighter.

BTW, the the fresnel should be on the lens side of the GG, with the rings toward the GG. The ground side of the GG should be against the frensel. I mention that because I have seen many Graphics with them put in any which way. Sorting that out is the first step in fixing focus problems with the cameras.