Optical condensers formed in wet-mounting setup

We reveal that a practical and simple sample arrangement for optical microscopy that is commonly used in biomedical imaging laboratories is a microscope condenser. This unnoticed but high quality microscope condenser is formed when the object under observation immersed in a liquid is sandwiched between two glass coverslips, and is observed using an oil-immersion objective lens. We demonstrate that the advantages in image resolution and contrast provided by this imaging arrangement come from the resulting microscope condenser. We also demonstrate that this overlooked condenser can be reconfigured as a variable numerical aperture microscope condenser by depositing a drop of low boiling point liquid on top of it. We present and discuss several experiments suggesting that the condenser-like rings observed in the Fourier plane images are formed when the incoming light bounces off, or gets scattered by the inner edge of the top aperture of the metal cage of the oil-immersion objective lens toward the top surface of the sample arrangement, and is either reflected, or totally-internally reflected back at a highly inclined angle toward the object under observation.