Philosphy

1590 Jansen invented the compound microscope1665 Robert Hook examined cork using microscope and used the term cellto describe its basic units.1650-1700 Antony van Leeuwenhoeckobserved nuclei in red blood cells of fish and unicellular organisms including bacteria(animalcules).1831-33 Robert Brown described nucleus in plant cells.1838-39 Schleiden(botanist) & Schwann(zoologist) proposed the cell theory.

1840Purkynegave the name protoplasm to the cell contents.

1855Virchow showed that cells arise from pre-existing cells.

1866Haeckel established that nucleus was responsible for storing and transmitting hereditary characters.1866-68 Cell division studied in detail and chromosomes discovered.1880-98 Chloroplast, Mitochondria, Golgi apparatus discovered.1930's Electron microscope discovered

1946Electron microscope became widely used in biology & ultrastructureof cells studied in more detail

Microscopy

Magnificationis the increasing in size of an object.

Resolutionis ability to distinguish between two separate objects.

Modern light microscopes use two magnifying lenses that act like back-to-back eyes.

The first lens focuses the image of the object on the second lens, which magnifies it again and

focuses it on the back of the eye. Microscopes that magnify in stages using several lenses are

called compound microscopes. They can resolve structures that are separated by more than

200 nm. Light microscopes, are not powerful enough to resolve many structures within cells.

Electrons have a much shorter wavelength, and a microscope employing electron beams

has 1000 times the resolving power of a light microscope. White hot tungsten filament is the source of electron beam. Transmission electron microscopes, so called because the electrons used to visualize the specimens are transmitted through the material, are capable of resolving objects only 0.2 nanometer apart.

A second kind of electron microscope, the scanning electron microscope, beams the electrons

onto the surface of the specimen from a fine probe that passes rapidly back and forth. The

electrons reflected back from the surface of the specimen, together with other electrons that

the specimen itself emits as a result of the bombardment, are amplified and transmitted to a

television screen, where the image can be viewed and photographed. Scanning electron