I didn't know there is such a thing as a metallurgical microscope. What makes them different from biological microscopes? And what is there primary purpose? I am assuming they don't make microscopes just for dissecting chips.
An external light works for something like an inspection microscope. But as you increase the magnification, you need something like a metallurgical microscope that focuses the light where you are looking. Otherwise, the image gets dimmer and dimmer as you zoom in.
*"Below" meaning "on the opposite side from the objective" - you illuminate _through_ the sample.
Biological microscopes, on the other hand illuminate the sample from the back side (which doesn't work for fully opaque objects).