Magnification is not the same as detail
Two ideas run through the whole history of the microscope. Magnification is how much larger an object appears. Resolution is how close together two points can be and still look like two points. An instrument can enlarge an image many times and still show only a smear if its resolution is poor. Most of the story below is about resolution, not about making things bigger.
The simple microscope
A simple microscope uses one lens. A strong convex lens held near the eye lets a person see a small object clearly at very short range. Hand magnifiers of this kind were in use long before anyone built a tube with two lenses.
The most famous maker of simple microscopes is Antony van Leeuwenhoek of Delft, who worked in the second half of the 1600s. His instruments were tiny: a very small lens, often a ground or fused glass bead, mounted between metal plates, with a pin to hold the specimen. The user held the plate close to the eye. Because a small, strongly curved lens has little room for error, a careful maker could get clear images at magnifications often estimated at a couple of hundred times. His methods were not fully recorded, and he kept some of them private, so exactly how he made every lens is still discussed.
Simple lenses have drawbacks. They are awkward to hold, tiring to use and limited in how strongly they can magnify while staying usable.
The compound microscope
A compound microscope uses at least two lens systems: an objective near the specimen that forms an enlarged image, and an eyepiece that magnifies that image again. Instruments of this type appeared in the early 1600s, and Robert Hooke used one for the drawings in Micrographia in 1665. The full story of who built the first ones is in the overview of early microscopes.
The arrangement is more comfortable to use and can in principle reach much higher magnification. In practice, though, early compound microscopes suffered from two optical flaws.
- Chromatic aberration: different colors focus at slightly different distances, producing colored fringes and blur.
- Spherical aberration: light passing through the edge of a lens focuses at a different point than light through the center.
Each added lens could add more of both errors. This is why a good single lens often beat a poor compound one for about two centuries.
Fixing the lens problems
Progress came from optics and manufacturing.
In the 1700s, opticians learned to pair different kinds of glass so that their color errors partly canceled, first in telescope lenses. Applying this idea to microscope objectives took longer. In the 1820s and 1830s, makers in several countries produced improved objectives, and Joseph Jackson Lister in England published an influential analysis around 1830 showing how combining lenses could reduce both kinds of error together. Achromatic objectives soon became standard in serious work.
Later in the 1800s, Ernst Abbe, working with the Zeiss workshop in Jena, put lens design on a calculated footing instead of trial and error. Abbe’s theory described how the wave nature of light limits what any lens can resolve. Special glass made in Jena, and the use of oil between the specimen and the front lens, improved sharpness further. This is the era in which the instrument became reliable enough for routine laboratory use.
The limit of light
Abbe’s work also made clear that visible light itself sets a boundary. Objects closer together than roughly a few tenths of a micrometer, about half the wavelength of the light being used, cannot be distinguished by an ordinary light microscope, however well it is built. Making lenses better could not cross that line.
Other light-based methods improved contrast rather than resolution. Dark-field illumination made small objects glow against a black background. Phase-contrast methods, developed in the 1930s by Frits Zernike, let people see transparent living cells without staining them.
Electron microscopes
The route past the limit was to replace light with electrons, which behave like waves with much shorter wavelengths. In the early 1930s, Ernst Ruska and Max Knoll in Berlin built a prototype electron microscope. It used magnetic coils as lenses inside a vacuum, in a design related to the electron beams of cathode ray tubes. Within a few years, instruments surpassed the resolution of light microscopes, and commercial models appeared in the late 1930s. Ruska shared a Nobel Prize for this work in 1986.
Two main types developed.
- Transmission electron microscopes pass electrons through a very thin slice of material to show internal structure.
- Scanning electron microscopes sweep a beam across a surface and build a detailed three-dimensional-looking picture of it.
Specimens usually must be dried, thinly sliced or coated, and observed in a vacuum, so living samples are not normally viewed this way.
Probes, not lenses
In 1981 Gerd Binnig and Heinrich Rohrer built the scanning tunneling microscope, which reads a surface using a sharp tip held extremely close to it, instead of using lenses or beams. Instruments in this family can map individual atoms, and they are central to the field now called nanotechnology.
Why the sequence matters
Each step solved the previous step’s limit.
- A single lens gave early detail but was hard to push further.
- Two lens systems allowed comfortable, higher magnification but at first added blur.
- Better glass and design theory made compound microscopes dependable.
- Shorter wavelengths, in the form of electrons, moved past the light limit.
- Scanning probes dropped lenses altogether.
None of these replaced the earlier ones entirely. A simple hand lens, a classroom light microscope and a research electron microscope are all still in use, because the right instrument depends on what a person needs to see.