By the early 1900's, optics in light microscopes were good enough to achieve the best resolution, so improvements since then have focused on improving contrast.
(optics in light microscopes; were; good enough to achieve the best resolution) (optics in light microscopes; to achieve; the best resolution) (improvements since then; have focused; on improving contrast)
optics in light microscopes optics in light microscopes improvements since then
were to achieve have focused
good enough to achieve the best resolution
the best resolution
on improving contrast