Evolutionary adaptations are the basis for the correlation between structure and function. The microscope's invention and improvement in the seventeenth century led to the discovery and study of cells. In 1665, Robert Hooke described cells using a light microscope . Modern light microscopy is based upon the same principles as microscopy first used by Renaissance scientists. Visible light is focused on a specimen with a condenser lens .
Light passing through the specimen is refracted with an objective lens and an ocular lens .
The specimen's image is thus magnified and inverted for the observer. Two important concepts in microscopy are magnification and resolving power . Resolution of a light microscope is limited by the wavelength of visible light. Maximum possible resolution of a light microscope is 0.2 m m. Highest magnification in a light microscope with maximum resolution is about 1000 times. 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.