Figure 1. Principle of imaging of the ocular lens. The light from the phase condenser converges when passing through the ocular lens. The former corresponding condenser annulus becomes unsuitable; therefore, we used a larger condenser annulus, Ph0, to match the 20× or 40× objective lens (Leica DMI3000 B) to obtain clear images of the downward-facing surface of the lens. The diameter of the Ph0 is lager than the diameter of the phase plate on the 20× or 40× objective lens.