Although at the level of consciousness we sense ourselves to be a single living entity, from a physical point of view, our material body is composed of between 50 trillion and 100 trillion cells. Each cell displays characteristics that biologists associate with life. Cells are, in fact, the smallest units composing us that are considered to be alive. Cellular parts (called organelles), and the large and small molecules that are found making up our cell parts (like carbohydrates, proteins, water, and vitamins) are considered non-living.
Human cells, like most cells in nature, are too small to be seen with our unaided eyes. The limit of good human vision is about 1/10 of a millimeter (which can also be expressed as 100 microns). An object at this size would appear as a tiny pin point, about ½ the size of the period at the end of this sentence. A human female's ovulated egg cell is about this size. But even if you knew where to look and saw the pin point-sized object, you could not make out any details; you would not know what you were looking at. Some plant cells and even some rare bacterial cells can also be just barely visualized with the naked eye. So the meaningful study of cells requires the use of an optical instrument - a microscope - which magnifies objects creating virtual, enlarged images that we can clearly see and study.
The living cells of animal and plant bodies are organized into groupings that perform coordinated functions. Such groups of similar cells are called tissues. Seeing blood is an example of seeing a tissue. Other examples include connective tissue, (e.g., the substance of bone), epithelial tissue (e.g., the surface layers composing our skin), and muscular tissue. The organs observed when a student dissects a laboratory plant or animal are actually clearly observable arrangements of tissues. For example your tongue is an organ. It has a surface covering of epithelial tissue; it contains muscular tissue which helps to give it movement; its nervous tissue is responsible for feelings of touch, pressure, heat and cold, and also the ability to taste chemicals in food; the connective tissues of the tongue hold it togehter giving it its familiar shape and appearance. The organ level of organization is clearly visible to the unaided eye. All levels smaller than this: cellular, organelle, molecular, and atomic are either totally invisible to us or barely visible.
The first microscope, actually a magnifying tube containing just one lens, is believed to have been invented by a Dutchman (whose name we do not know) in the late 16th century. The Italian scientist, Galileo (1564-1642), constructed his version of this magnifying tube about 1609 and used it as a 32 power (32X) microscope to study insects and also as a telescope to observe the heavens. Galileo sent one of his magnifiers to the German Astronomrer, Johann Kepler (1571-1630), who used it as a telescope. Kepler also drew up plans for making a more powerful microscope having more than one lens in the tube (a compound microscope). Kepler's plans, and probably those of other observers, became more widely known during the 1600's and microscopy became extremely popular in scientific circles.
Anton van Leeuwenhoek (1632-1723), a Dutch microscopist, was probably the most interesting of the early observers of microbiological life. Using simple microscopes of his own design, he achieved useful magnifications of up to 200X and discovered protozoa, bacteria, and gave the first discription of sperm. His communications with England's Royal Society, and its secretary, Robert Hooke, further stimulated microscopic investigations. Robert Hooke used both his own compound microscopes and simple magnifiers of Leeuwenhoek's design. Hooke coined the word "cell" as he described the tiny chambers in the cork of tree bark.
Cells were identified in many living things by many observers over the next 150 years. By 1838, Matthias Schleiden (1804-1881), a German botanist put forth a theory, based on the work of many scientists, that all plants were composed of cells as their living units. In 1839 Theodor Schwann (1810-1882) broadened the concept of a cell theory to include animals as well as plants. Cell theory states, further, that all cells come from previously existing cells by a process of cell division (mitosis, which will be taken up in another exercise).
Early microscopes were unable to produce truly clear images. The images were distorted due to the differential bending of light of different wavelengths through optical glass (chromatic abberation), and due to variations in the exact focal point of light as it passes through lenses that are curved (spherical abberation). Such abberations limited the useful magnification of early microscopes to a maximum of about 200X. Today's compound light microscopes correct for both chromatic and spherical abberation and the maximum useful magnification is now about 2000X. This limitation is based on the size of the wavelengths of visible light. The shortest wavelengths are capable of producing the greatest magnifications. Since blue/violet light has the shortest wavelengths of all visible light, the greatest magnifications are achieved using filters over the light source that only allows light from the blue end of the spectrum to illuminate the objects. The electron microscope is capable of much greater magnifications due to the extremely short wavelengths associated with the electron beams used.