The development of the microscope, along with myriad other inventions, was a direct consequence of mankind's innate curiosity.
We humans have always been fascinated by the world around us. First we looked up and set our gaze upon the sun, moon and stars. We then took an interest in the objects, large and small, that lay closer to hand. Sometime in our dim and distant past, a curious individual found that an object appeared larger when viewed through a convex (curving outwards) rock crystal; The oldest lens artifact has been dated to the 7th century BC. Our ancestors also found that these primitive lenses could create fire by focusing the sun's rays; Both the Greeks and Romans were familiar with these 'burning glasses'.
But we must move forwards at least a millennium before we find any significant developments in the 'magnifying glass'. When a Florentine called Salvino D'Armate invented the first 'reading glasses' in 1284 (this claim has been disputed), experimentation began, to explore the magnification possibilities of lenses. These early microscopes were simple cylinders with a lens at the top, and their magnification could not exceed 10 diameters, or, x10.
The first major development in magnification came with the work of two Dutch spectacle makers, Zaccharias and Hans Janssen. In their experiments with several lenses grouped together, they paved the way for what would become the 'compound microscope' (The great Galileo would subsequently 'borrow' and improve their idea).
A compound microscope consists basically of two lenses and is capable of greater magnification than a single lens. One lens (the objective) is put close to the object of scrutiny. Another lens (the eyepiece) is used to examine the image formed by the first lens. Both the lenses are fixed in the ends of a cylinder which cuts out all the light not coming from the viewed object.
Another Dutchman, Anton van Leeuwenhoek, took the microscope a stage further in the 17th century. In his work as a draper he was required to count fabric threads with a magnifying glass. This stimulated his interest, and he began to experiment with, and manufacture, lenses. He is known to have constructed over five hundred microscopes in his lifetime, although few still exist. He was also thought to have been inspired by a popular illustrated book of the time, 'Micrographia', written by the Englishman Robert Hooke. Van Leeuwenhoek's work made magnifications of up to 250 diameters (x250) possible, and detailed studies began to be made of truly 'microscopic' subjects, such as sperm cells, blood cells and bacteria.
By the 19th century, precision lenses were being manufactured which permitted magnifications up to 1250 diameters. Yet the development of the microscope was approaching a barrier. The problem lay in the limitations of light itself.
Modern 'light' microscopes can give a maximum magnification of about x2,000. Any magnification above this and the object becomes too blurred to be recognizable. This is because light microscopes cannot distinguish objects smaller than half the wavelength of light (0.275 micrometers). Smaller than that and objects are all but invisible, and two tiny particles become indistinguishable (the ability to distinguish them is called a microscope's 'resolving power'). What was required was a means of 'lighting' the object of scrutiny without using light.
The answer came with the invention of the electron microscope in the 1930's. Instead of using light, a stream of electrons (of extremely short wavelength) from an electron source, passes through the object. The stream is spread out as it passes between the poles of an electromagnet, and a greatly magnified image is formed on a photographic plate. Extreme magnifications are now possible.
But the electron microscope has a major drawback. Because the object is in a vacuum (to speed up the electrons and reduce their wavelength) no living matter can survive. Therefore it is impossible to study the structures of living cells. Yet this can now be done by a polarized light microscope. As well as examining living cell structures they are also used to study the stresses and strains in industrial materials.
As our abilities evolve we can only wonder how far into the heavens we will see, and how far inside our very matter we will delve. What has not changed, though, is our irrepressible curiosity. As our cro-magnon ancestors gazed quizzically upwards, they exhibited a curiosity no more nor less than our own. Yet we can now SEE so much further. Where, indeed, can it end?