Top Lighting vs Bottom Lighting
Bottom lighting and top lighting refer to the source of light you’re using on your specimen. The light from the bottom of your microscope is always a bottom light (because it comes from the bottom). If you were to use a lamp or other source of light to shine on top of the specimen, those lights would be called toplights.
Setting Up Top Lighting
All you need to set up a top light is a lamp or other fixed source of lighting. Once you’ve found a suitable source, just make sure that the light is as close as possible to the stage of your microscope. I personally use a normal reading lamp for top lighting.
Advantages and differences
Top lighting helps you see specimens that don’t allow light to pass through. Things like shells, thick leaves, and insect exoskeletons would all require a top light. Bottom lighting would best be used for transparent or semitransparent specimens like crystals and insect wings.