Cell theory and microscopes
Most cells are too small to see without a microscope (a few, such as some egg cells, are visible), so the discovery of cells followed the invention of the microscope. In 1665 Robert Hooke looked at thin slices of cork and named the tiny compartments he saw cells, and in the 1670s Antonie van Leeuwenhoek used his own powerful lenses to observe living single-celled organisms. By the mid-1800s, scientists including Matthias Schleiden, Theodor Schwann, and Rudolf Virchow had pieced together the cell theory: all living things are made of one or more cells; the cell is the basic unit of structure and function in living things; and all cells come from cells that already exist. A compound light microscope uses two sets of lenses to magnify specimens up to about 1,000 times and can show living cells, often with stains added to make structures easier to see. Electron microscopes use beams of electrons instead of light and can magnify far more, showing details such as the inside of organelles, but their specimens must be specially prepared and are usually not alive. Most cells are measured in micrometers, millionths of a meter.
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