History of the microscope

The history of the microscope is a history of limits. The first was the quality of glass, then the wavelength of light, then the wavelength of the electron and the damage that an electron beam does to what it looks at. Each was met by a new kind of instrument, and the newest instruments no longer form an image in the old sense. A scanning tunneling microscope draws a surface from a current; a Coulter counter counts cells from dips in a current and has no image at all. The change from showing a thing to measuring it runs through the whole history, and it ends in the nanopore, which reports the passage of one molecule as a number.

A single lens will magnify, and the earliest microscopes were one. The compound microscope, in which a second lens enlarges the image made by the first, is traditionally credited to Dutch spectacle makers of the 1590s. In 1655 Willem Boreel, then the Dutch envoy in France, testified that his boyhood friend Zacharias Janssen of Middelburg had invented it. The testimony came sixty years after the event and from a friend, and historians treat the claim with caution.^[1]

The first great observer was not a maker of compound instruments. Antonie van Leeuwenhoek of Delft ground single lenses a millimeter or two across, held them in a metal plate and in the 1670s described bacteria and protozoa to the Royal Society, which at first doubted him.

In England Robert Hooke used a compound microscope and drew what he saw in Micrographia (1665), among other things a flea and the walls of cork, which he called cells. Samuel Pepys sat up until two o'clock one January night that year reading it and wrote in his diary that it was "the most ingenious book that ever I read in my life".^[2] Hooke was asked by the Royal Society to repeat Leeuwenhoek's observations, and in 1677 he showed the Fellows the small animals in pepper-water.

Makers spent two centuries improving the glass. Compound lenses of the seventeenth and eighteenth centuries gave colored fringes and soft edges, and the aberrations were corrected in the nineteenth century, by trial and then by calculation. In 1873 Ernst Abbe in Jena published a theory of how a microscope forms an image, and it showed that however good the lenses, resolution is limited by the wavelength of the light and the angle of the cone it is collected in. In practice the limit is about half a wavelength, 0.2 micrometer, which means that whole cells and some of their parts can be told apart and a virus or a single protein cannot.^[3][4] Abbe's calculations were used at the firm of Carl Zeiss, and with the new glasses of Otto Schott they produced the apochromatic objectives of 1886.

The limit applies to images, and the particles themselves may still be detected. In 1903 Henry Siedentopf and Richard Zsigmondy described the ultramicroscope, in which the sample is lit from the side against a dark field. A particle smaller than the resolving limit scatters light into a point that can be seen though not shaped, and by counting the points in a known volume of known mass concentration one can work out the size of the particles. Zsigmondy used it on the gold particles in ruby glass, and received the 1925 Nobel Prize in Chemistry for demonstrating the heterogeneous nature of colloidal solutions.^[5][6] Jean Perrin counted and tracked grains of resin under a micrometer across. In the preface to Les Atomes he remarked that a magnifying glass or a microscope leaves one just as uncertain about the outline of a soap flake, because every increase in magnification brings new irregularities into view.^[7]

Contrast came later than resolution. Living cells are nearly transparent, and Frits Zernike's phase-contrast microscope turned small differences in the speed of light through the specimen into differences in brightness. He received the 1953 Nobel Prize in Physics "for his demonstration of the phase contrast method, especially for his invention of the phase contrast microscope".^[8]

An electron has a wavelength far shorter than that of light. Ernst Ruska, a student at the Technical University of Berlin in the late 1920s, worked with his supervisor Max Knoll on magnetic coils as lenses for electrons. Two coils in series gave a magnification of fifteen times, which the Nobel committee called the first prototype of an electron microscope. In 1933 Ruska built one that outperformed a light microscope, and the first commercial instrument, made by Siemens, reached the market in 1939. Theory permits a resolution of 0.5 to 1 ångström at the voltages in normal use, and in practice about 1 Å has been reached.^[9]

The price was the specimen. An electron microscope needs a vacuum and a thin section of the object, and the preparation kills a cell.^[4] The 2017 Nobel Prize in Chemistry went to Jacques Dubochet, Joachim Frank and Richard Henderson "for developing cryo-electron microscopy for the high-resolution structure determination of biomolecules in solution", in which the sample is frozen fast enough to keep its water.^[10] Electron microscopy has also been proposed as a way to read DNA; see Electron microscopy DNA sequencing.

In 1981 Gerd Binnig and Heinrich Rohrer built a microscope with no lens. A stylus ending in a single atom is held about two atom diameters from a surface, and a voltage between them drives a current across the gap by the tunnel effect. Because the current depends strongly on the distance, a feedback circuit that holds it constant moves the tip up and down with the atoms, and the record of the movement is the image. The Nobel committee, which gave Ruska one half of the 1986 physics prize and Binnig and Rohrer the other, wrote that the instrument "is not a true microscope (i.e. an instrument that gives a direct image of an object)".^[9] Erwin Müller's field ion microscope had already resolved individual atoms in 1955.^[11]

In 2014 the Nobel committee said that Abbe's equation still holds "but has been bypassed just the same". Stefan Hell proposed in 1994 to shrink the glowing spot in a fluorescence microscope by switching off the fluorescence around its center with a second beam of light.^[12] Hell, Eric Betzig and William Moerner shared the Nobel Prize in Chemistry "for the development of super-resolved fluorescence microscopy".^[4]

Until the middle of the twentieth century blood cells were counted by a technician with a microscope and a ruled slide, a task of 15 to 30 minutes. A count of the few hundred cells that fit on the grid was good to about 16 percent for red cells and 21 percent for white cells.^[13] Wallace H. Coulter replaced the grid with an aperture and an electric current; the patent, US 2,656,508, was granted on October 20, 1953. Its transistorized Model F still projected an image of the aperture on a screen, in the place where the microscope had been.^[14][15]

Molecular Reality Corporation has taken the same step with a nanopore. Its public guide says that "the nanopore does not look at the molecule. It feels it, electrically, the way your fingertip feels the edge of a page."^[16]

• Antonie van Leeuwenhoek
• Micrographia
• Brownian motion
• Coulter principle
• Evolution of the eye
• The Seeing Eye