A mirror illuminator is an alternate, non-electric light source for your microscope.
It is particularly handy for using your microscope in places where you have no access to electricity. The fluorescent bulb is more reliable than the mirror illuminator, but the mirror illuminator works wonderfully in outdoor situations where any type of electric bulb would be worthless.
To install the mirror illuminator in your Sonlight Ultra microscope (or any other microscope we sell), simply unscrew the Abbe condenser (the cloudy lens directly beneath the mechanical stage) and install the mirror stand in its socket. The mirror itself snaps into a U-shaped holder that allows you to swivel the mirror in the direction of the brightest light.
Easy to install, and well worth the money if you plan to do a lot of outdoor field work.