Think of what limitations are put on us by the restrictions of our own eyesight: things too far away, or too small, are beyond our ability to observe without some sort of devices to help. Just as the telescope was a critical device for astronomers, the microscope was critical to the progress of biology.
Although magnifying spectacles (glasses, sort of) have been in widespread use since the 1300's, the use of lenses to see very tiny objects was a slowly-developing technology. First of all, the tinier an object is, the less light reflects off it or passes through it, and seeing anything really tiny requires a decent illumination system. Secondly, magnifying lenses used in early microscopes were made of glass that was not particularly even or clear, and tended to split light like a prism, which affected the resolution limits of the lenses. Resolution can be thought of as the clarity of focus; technically, it is the limit at which two tiny objects which are close together stop being visibly separate. The resolution of early microscopes was very limited by the glass used in the lenses.
Some important work was done with early microscopes, although they were primarily more of a toy than a scientific instrument. Like telescopes that were being developed through that same period, many early microscopes used lenses in sequence (making them multi-lens or compound microscopes) to magnify an image. In 1660, Italian Marcello Malpighi was able to use a microscope to see blood capillaries in the tail of a fish, providing powerful supporting evidence that blood circulates in the body (prior to this, since no one knew of how blood runs from arteries through capillaries to veins and back, it was thought that the blood flow was one-way from production in the intestines to consumption in the body tissues). In 1665, Englishman Robert Hooke found that cork was full of tiny chambers, which he called cells (there were no actual cells as they are now known in the dead cork, but our label did come from his label). In the 1670's, Dutchman Antony van Leeuwenhoek, using a special, especially pure single lens (a simple microscope) placed in a holder and held up very close to the eye, was the first person to write extensively about a world of tiny independent creatures, which he called animalicules, which seemed to exist all around but were too small to see by eye. Imagine what an odd idea that must have been to the people of the day!
It wasn't until the 1800's, through a interesting development period that included instances of thievery, plagiarism and questionable patent ethics, that many of the distortions of the lenses were corrected. With the technological limitations for microscopes mostly solved, by the end of that century microscopes had begun to hit resolution limits set by physics: to oversimplify, light beams themselves have a physical size, and when a gap is at or below about 0.2 micrometers the light itself can no longer fit through that gap - you can't see the gap, just a smudgy blob where the light has been blocked. Objects below that size just could not be resolved as long as light was used in the imaging system. Light microscopes (called that because they use light as an imaging system) would always remain useful, but most of the objects discussed in the next section on cell structures were invisible to them. Although there have been some recent techniques in improving light microscopes abilities, they are still limited.
Through the middle of the 1900's, a new system was developed that could use a beam of electrons, which have an adjustable beam much smaller than visible light beams. Electron beams can be narrowed to below the size of atoms. The beam is focused with magnets and the final image converted to light in a way similar to how television screens work. Recent versions of
electron microscopes have been used to produce images of molecules and
atoms. Electron microscopes are more expensive and complicated than light microscopes, and the beam needs to travel through a vacuum to avoid scattering off air atoms. They are very useful in research but not going to appear in classroom teaching labs any time soon.
There are two basic set-ups that affect the type of specimen that can be viewed and what the final image looks like; both set-ups can be found in both light and electron microscopes. In a typical laboratory class microscope, light must pass through a specimen to reach the eyes of the viewer; this set-up is an example of a transmission microscope. For a specimen to be useful in a transmission scope, the beam needs to be able to pass through it and reveal its internal details. Specimens may need to be stained, colored or darkened with special dyes, to produce clear details. Colored dyes are used in light microscopy (the use of microscopes is called "microscopy"), while electron microscope stains have heavy metals in them (the images are black-and-white, so the stain just needs to produce variations in electron absorption; colors may be added later to make some details clearer). Obviously, large specimens or structures are not going to let the beams through them; to see large specimens, the original object must be sectioned, converted into very thin slices, then stained for detail. Objects are embedded in a material that will hold them together (commonly wax for light microscopy, plastic for electron microscopy) and then thinly sliced. The ability to translate the two-dimensional information of sections into three-dimensional concepts is important in microscopy.
The other set-up involves reflecting the beam off the surface of the specimen, which creates a much more three-dimensional image. This is done by scanning microscopes, named for the necessity in the electron version of scanning the surface with the electron beam. A dissecting microscope is a type of scanning scope.