I am trying to create a mob farm in my custom flatworld, and was trying to prevent light getting into it so mobs would spawn, and place a spruce slab on half the block so I can still see in and kill them, but the slab isn't lowering the light level. In the picture you can see half of the block covered which is letting light in. Also its in 1.2.2 but it won't let me input that as a valid version