Which instrument is used to look at things far too small to see?

A microscope magnifies objects too small for the naked eye. A simple lens can only do so much, but placing two lenses in line, an objective near the specimen and an eyepiece near the eye, lets the second one magnify the image the first has already enlarged, which is what pushed magnification into the hundreds. Optical microscopes eventually hit a hard limit set by the wavelength of visible light, so objects much smaller than a few hundred nanometres cannot be resolved no matter how good the glass. Electron microscopes get around this by using beams of electrons, whose effective wavelength is far shorter, revealing viruses and even arrangements of atoms. The instrument transformed biology: it revealed that tissue is built from cells, and later that many diseases are caused by microorganisms, which reshaped medicine and hygiene. Early observers in the seventeenth century were the first humans to see bacteria and sperm cells.

Both microscopes and telescopes trace back to the same burst of lens-making in the Netherlands around 1600.