Oh well than you can't do that with transparency. You have to use Boolean functions to do things like that. There is a tutorial to on how to make something like that. Page 255 of the book. This is the only way to do it with meshes. barring making the mesh in some different progam but even then it will probably use this method unless it can extrude complex shapes. Remember if your making a house say you need two cubes one for the out side and a slightly smaller one for the inside to hallow it out then windows and doors will cut out the solid wall letting the light shine in. It's all done with Boolean functions.