Microscope

A microscope uses one or more lenses to show things too small for the eye to see. Compound instruments with two lenses in a tube appeared in the Netherlands around 1600, and in the 1670s Antoni van Leeuwenhoek's single tiny lenses showed bacteria and single-celled life for the first time.

What it is

A microscope shows details too small for the unaided eye to make out, from a tenth of a millimetre down to a few thousandths of one. The simplest kind, the simple microscope, is a single strong lens held close to the eye. The compound microscope uses two lenses or groups of lenses in a tube: the first makes an enlarged image and the second enlarges that image again.

The instrument on the plate is a compound microscope of the type the London instrument maker Edward Culpeper developed around 1725 to 1730. Three curved brass legs on a wooden base carry the stage; three more carry the sleeve in which the body tube slides up and down for focusing. A concave mirror below the stage gathers daylight or candlelight and sends it up through the specimen.

How it works

The specimen, often a thin slice or a drop of liquid on a glass slide, sits over a hole in the stage. The mirror throws light up through it. Just above the slide, the objective lens, small and strongly curved, forms a real, enlarged and upside-down image of the specimen inside the tube. The eyepiece at the top works like a magnifying glass on that image. In the plate, a field lens just below the eye lens widens the area that can be seen at once.

The two enlargements multiply. An objective that magnifies ten times and an eyepiece that magnifies ten times give a hundred times in all. Moving the tube up or down brings the specimen into focus; at high power the depth that is sharp is only a few thousandths of a millimetre.

Leeuwenhoek’s instrument, seen from the side: the long screw raises the specimen pin, the short one pushes it toward or away from the lens.

More magnification does not always show more. Early lenses bent each colour by a slightly different amount and blurred the image with coloured fringes. Joseph Jackson Lister solved much of this around 1830 with objectives that combined two kinds of glass. Later in the century Ernst Abbe, working with the Zeiss firm in Jena, showed that the wave nature of light itself sets a limit: a conventional light microscope cannot separate two points much closer than about 0.2 µm, roughly half the wavelength of visible light, and the exact figure depends on the colour of the light and the aperture of the objective. Smaller things need electron microscopes or special fluorescence methods.

How to build it

A single-lens microscope of Leeuwenhoek’s type is within reach of a careful amateur and can reach 100× or more.

Make the lens. The easiest lens is a glass or sapphire ball 2 to 3 mm across, sold for optics or for fibre couplers. A ball 2 mm across made of ordinary glass has a focal length of about 1.5 mm, measured from its centre, and magnifies about 150 times. The specimen has to sit only about half a millimetre from the glass surface. Leeuwenhoek also made lenses by melting glass; heating the tip of a thin glass rod until a round bead forms works, but it needs a gas flame and is best left to people trained in lampworking.

Mount it. In two thin strips of brass or aluminium about 5 by 2.5 cm, drill a hole about half the diameter of the ball, which serves as the aperture and cuts out the blurred edge of the lens. Countersink the inner side of each hole so the ball sits in it, sandwich the lens between the strips and rivet or bolt them together near the edges. File off every burr and round the corners.

Add the specimen holder. Fix a small block behind the plate on a long screw passing through a bracket, so that turning the screw raises and lowers it. Fit a pin, or a glass coverslip with finished edges, to the block to hold the specimen just behind the lens, and a second screw through the block, pressing on the plate, to adjust the distance. Use fine threads: the specimen must be brought to within half a millimetre of the lens and moved by hundredths of a millimetre.

A drop of pond water seen at about a hundred times: paramecia, a Volvox colony, diatoms and a green alga.

Use it. Hold the lens close to your eye without touching it, with the specimen on the far side, and look toward a bright, diffuse light such as a white wall or an overcast sky. A drop of pond water on a coverslip will show moving protozoa and algae.

Safety. Never point any microscope or its mirror at the Sun: concentrated sunlight can cause permanent eye damage in an instant. Hot glass looks the same as cold glass, so let beads and rods cool on a heatproof surface, and keep gas flames away from anything flammable. Wear eye protection when drilling metal. Treat pond water as unclean and wash your hands after handling it.

History and evolution

Spectacle makers in the Netherlands were the first to put two lenses in a tube, around 1590 to 1620. Later claims credited Zacharias Janssen of Middelburg, but they rest mostly on statements made decades afterwards by his son, and Cornelis Drebbel and others built similar instruments. In 1624 Galileo sent one of his “occhialini” to Federico Cesi, and the next year Johannes Faber, a member of Cesi’s Accademia dei Lincei, proposed calling it a microscope.

Hooke’s microscope of the 1660s: a globe of water gathers the light of an oil lamp onto the specimen.

Robert Hooke’s Micrographia, published by the Royal Society in 1665, made the compound microscope famous. Its thirty-eight engravings showed a flea, a louse and a thin slice of cork, whose empty compartments Hooke called cells. In Delft the cloth merchant Antoni van Leeuwenhoek saw finer detail with a single lens. He made several hundred small microscopes, some magnifying more than 250 times, and from 1673 sent letters to the Royal Society describing blood cells, sperm and swarms of “little animals”. In a letter of 1676 about water in which pepper had been steeped he described organisms now recognised as bacteria, the first ever seen, and in 1683 he found similar ones in the plaque from his teeth.

The compound microscope only overtook the simple lens in the nineteenth century, when Lister’s achromatic objectives and Abbe’s theory turned it into a precise laboratory instrument. Electron microscopes, built from the 1930s, look far below the limit of light, and since the 1990s some fluorescence methods have let optical microscopes get past it.