Some cells are visible to the human eye. The smallest objects that the unaided human eye can see are about 0.1 mm long. That means that under the right conditions, you might be able to see an ameoba proteus, a human egg, and a paramecium without using magnification. A magnifying glass can help you to see them more clearly, but they will still look tiny. Smaller cells are easily visible under a light microscope. It's even possible to make out structures within the cell. Light microscopes use a system of lenses to magnify an image. The most powerful light microscopes can resolve (form images of) bacteria but not viruses. To see anything smaller than 500 nm, you will need an electron microscope. The most powerful electron microscopes can resolve (form images of) molecules and even individual atoms. From: http://learn.genetics.utah.edu/content/begin/cells/scale/
MEET THE SCIENTISTSDEVELOPING THE CELLTHEORY
WHAT IS A CELL? The cell is the basic unit of structure and function in living organisms, including plants and animals.
TOOL USED TO VIEW CELLS The microscope is used to magnify extremely small objects.
TIMELINE - 1590 Hans and Zacharias Janssen (1590) Dutch lens grinders Father and son Produced first compound microscope (2 lenses)
TIMELINE - 1665 Robert Hooke (1665) English scientist Looked at a thin slice of cork through a compound microscope and observed tiny, hollow room-like structures. He called them cells, but he only saw the outer cell walls because the cork cells were not alive.
TIMELINE - 1680 Anton van Leeuwenhook (1680) Dutch amateur scientist Looked at blood, rain water, and scrapings from teeth through a simple microscope. He called the structures he saw “animalcules”.
TIMELINE - 1838 Matthias Schleiden (1838) German botanist viewed plant structures under a microscope and discovered that they were made of cells.
TIMELINE - 1839 Theodor Schwann (1839) German zoologist viewed animal tissue taken from animal organs under a microscope and discovered the tissue was made of cells.
TIMELINE - 1855 Rudoph Virchow (1855) German physician Stated that living cells come only from other living cells (This is a part of cell theory.)