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Any gizmo that can be used to inspect small regions of space with the help of optics - for example a single magnifying lens - can be called an optical microscope. In fact, primitive microscopes were nothing else but magnifying lenses mounted on a holder. Along with the evolution of optical science, particularly in the mid-17th Century, better and better lenses were being manufactured. And soon the classic compound microscope was invented. Shortly after that, the microcosm was subject to the human eye.
The Development of Microscopes
The first (compound) microscopes were built in the end of the 16th Century by Zaccharias Janssen and his father Johannes. Both were fooling around with two lenses in a tube in the basement of their eyeglass workshop in Middleburg, Holland. They found out that they could improve the magnification if they assembled the lenses so that the distance between the lenses is smaller than the focal length of the stronger lens. Voilà!: The first compound microscope was built.
In the 17th Century, the best lenses of the world were being manufactured in Holland. For that reason, it's no surprise that another Dutch man by the name of Anton Leeuwenhoek (1632-1723) later became known as the 'father of microscopy'. He was the first person able to see and describe cells and micro-organisms. All with his self-made two-lens microscope.
Throughout the 17th Century, a huge science-occultism frenzy was sweeping through the upper classes of Europe1. A group of science freaks called Academia del Lincei (one of the most notorious members being Galileo Galilei) was, obviously, playing with these new magnifying gizmos. However, they did not come up with any extraordinary contribution to the field of microscopy, except perhaps for the coining of the word 'microscope'.
Another science weirdo of his time was Englishman Robert Hooke, who was also caught by the microscopy fever. Hooke had the remarkable ability to improve every bit of science he laid hands on. In microscopy, he improved Leeuwenhoek's design by adding a third lens behind the original two, which made the usage of microscopes a lot more comfortable. The normal two-lens design required the viewer to look into the tube from a relatively long distance, which caused two problems: First the viewer had to aim very well and afterwards keep his head still. Second, the images of the 2-lens design were dim due to low brightness and contrast. The third lens improved the brightness and contrast problem and allowed a more comfortable (nearer) working distance. The modern microscope is still based upon Hooke's three-lens design, in which the original three lenses are replaced by whole multi-lens and mirror arrays to improve even more the quality of the images.
The discoveries made by the early microscopists started to revolutionise the way people thought about life. The first two important books were published in 1660 by Italian Marcello Malpighi, who proved William Harvey's blood circulation theories (by proving the existence of capillary blood vessels, which were until then unknown), and in 1665, Robert Hooke's Micrographia, where the word 'cell' is used for the first time2. However, the common notion of the microcosm at that time was that it's merely a miniaturised version of what we see in our macroscopic world. It took another while until the microscopic world got understood.
Problems in Microscopy and How They Got Solved
In the 18th Century, the design of the microscope had reached a level of perfection. There were some unsolved problems though: No matter how perfectly manufactured, at some magnification the lenses ended up providing blurry images. Eventually some scientists found out the reasons for this. First, a lenses capacity to bend light depends on the light's wavelength. For that reason, red light is focussed on a slightly different plane than blue light. This causes blurry rainbow coloured halos around the inspected objects, an effect called 'chromatic aberration' (from 'chromos' which is Greek for colour). Second: The shape of the lenses is derived from a sphere, meaning that the focus is only perfect for beams going through the middle of the lens. Beams going through the edges cause the image to get blurry. This kind of blur is called 'spheric aberration'. However, if one blocks the edges to avoid spheric aberration, the images do also get blurry because of the low angular aperture causing interference effects (see below).
The chromatic aberration problem got solved around 1735 by Chester More Hall. He theorised that by using a combination of concave and convex lenses made out of different materials (crown and flint glass) but with the same curvature, the chromatic aberration could be compensated without losing magnification power. Before getting the patent, he had to make sure his gadget worked - he cleverly recognised the monetary potential of his invention. He was careful enough to order the lenses in two different shops, but both bought the lenses from the same lens-maker, George Bass, who put 1 and 1 together and figured out what Hall was up to. However he decided to keep silent for a while. Strange enough, Hall never published his work, and 20 years later (Hall was dead already) Bass told John Dolland, who was a telescope maker, the secret. John Dolland quickly checked out if it worked by building his own set of so-called 'achromatic lenses'. He got it patented even quicker and became obscenely rich by selling 'achromatic' telescopes. For microscopes, though, it took another half-century until achromatic lenses could be manufactured with satisfactory quality.
The spherical aberration problem got solved by Joseph Jackson Lister in 1830. By that time, it had been shown that low curvature lenses would cause less spheric aberration, at the cost of low magnification power. However, a clever arrangement of weaker lenses would yield higher magnification without adding up the spherical aberrations, so in the end the aspheric lens (array) was created. Again, this technology was first implemented for telescopes, and almost 20 years passed until aspheric lenses for microscopes became manufacturable.
The Resolution Limit: As Good as it Gets
With these new achromatic and aspheric lenses, people thought it was just a question of manufacturing skills to get better (read: more magnification) microscopes. In 1877, Ernst Abbé, who was working for Carl Zeiss, showed that there is a ultimate limit for the resolution given by the angular aperture: No matter how much one could magnify stuff, in the end what really counts is resolution3. In summary, the resolution is the limit, and it is given by the wavelength of light divided by twice the numeric aperture (which is in any case a number with a value close to 1). The numeric aperture is in its turn related to the distance of the focal spot from the lens and the lenses width. A more detailed description of this 'resolution limit' can be found in the entry on optical microscopes.
The 20th Century
Few major improvements were made after the last half of the 19th Century; microscopes remained expensive techy gadgets for a very restricted community of scientists. By that time topnotch microscopes were being manufactured by Powell & Lealand and R & J Beck in England, by Charles A Spencer in the United States, and, in the end of the 19th Century, also by Carl Zeiss and Leitz in Germany. Present day instruments changed little in design, and eventually became cheaper as more cost-effective production techniques for lenses were developed. Today microscopes are an essential tool in every high school, college, hospital, and research laboratory. To meet the special needs of scientists and industries, startling new variations have been developed, like: Fluorescence microscopes, phase difference and various types of confocal microscopes.