Do I understand your image correctly? You used 2 "shells" with each shell
created as the difference of a sphere and a smaller sphere. You then made
two CSGs, each out of two of these shells, one a Union and one a Merge. And
you then made two more CSGs (differences this time) with the two former CSGs
and some kind of cube, to "cut" the shell-CSGs open to view the interior (or
something like that).

Well, I assume this is how you produced the image, so I try to reply with
this in mind. Only it is not to easily to explain in writing (at least not
for me :-). Here it goes:

Do not confuse the interior of a POV-Ray primitive with the "inside" of a
more complex object you create with a CSG. The interior of a box or sphere
for example is well defined. The "inside" of a CSG object (or at least, what
the user sees as the inside) may be something totally different, as in your
case:
What you interpret as the "inside" of your "shell" (the "hollow" part inside
the shell) is not the interior of the POV-Ray object. Instead for POV-Ray,
the interior of such a shell is the shell-part itself: The "blue" part in
your image. Since Merge only makes a difference regarding the interior of
the object as POV-Ray sees it (not as the user may percieve it
"intuitively"), it has nothing to do with making your second object "empty".

Somebody correct me if I am wrong, but I think you can only see a difference
between a Union and a Merge, if at least one object in the CSG has a at
least partially transparent texture, so that you can see "into" the object.
In my experience Unions render faster than Merges (is that true?) so the
only reason to use a Merge would be, when you have a CSG with transparent
objects and you don't want to see interior surfaces.

Try this: Make a CSG out of two normal spheres that intersect each other.
Assign some glass texture to the CSG (without refraction). Now watch the
difference between Union and Merge: With the Union, you can see the outline
of each sphere inside the other (through the glass). With the Merge both
objects become one: There are no interior surfaces to see.
Now do the same with your "shells" (only use a non refractive glass texture
now): You will see that now the shells continue inside of each other again.
This is because the "empty/hollow" part of the shells is not the "interior".
Well, it is "inside" of the shell, but for POV-Ray it has the same status as
any other place in 3D space "outside" of the shell: It is space occupied by
only one object (or none at all), so there is no need for any special
Merge-CSGing. But if you look closely at the shell's shell itself (what is
blue in your image, and should be "glassy" now) then you will again see a
difference between Union and Merge.

Pewh, hope it helps :-)

Johannes.

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