duno i like cookies ;)
A flart is a cross between a flan and a tart.
eat a flart head or fly around like a chicken saying bok-bok bokkak!
That is a plano-concave lens. Its use in a microscope is usually to avoid chromatic distortions (rainbow effects). Using a glass for the convex lenses which has a weak rainbow (color dispersion) balanced by the occasional concave lens with a strong dispersion, we can cancel most of the dispersion while still, on balance, magnifying. This problem is always present in lenses. Look up the use of "crown" and "flint" glasses in lens design.