Microscope

The first optical microscopes produced images through the use of visible light. These microscopes used drops of water captured in a small hole to function as a magnifying lens. Later, magnifying glasses consisting of a single lens that was bowed outward on either side (biconvex lens) were developed. The lens was capable of enlarging an object up to 20 times its original size as seen by the naked eye. (Scientists describe the power of magnification by writing a number followed by a times sign "x". Thus, a 20 times magnification would be expressed as 20x.)

Using several lenses in conjunction with each other is the secret of a microscope's success, however. Dutch spectacle maker, Zacharias Janssen, devised the first compound microscope (a microscope that contains more than one magnifying lens) in 1590. Today's basic optical compound microscopes consist of a two-lens system with an objective and an ocular lens, and can magnify up to 2,000x. The specimen sits close to the objective lens, which magnifies the images as an ordinary magnifying glass would. The lens forms an enlarged real image of the specimen near the ocular or eyepiece lens. The eyepiece works in conjunction with the objectives to correct for aberration and further magnify the image, creating the virtual image seen in a microscope. Light reflected from a mirror and concentrated by the condenser lenses located under the specimen stage, illuminates the object.

One of the early enthusiasts of microscopy was Antoni van Leeuwenhoek. During the sixteenth century he constructed over 500 single lens microscopes. While crude by today's standards, van Leeuwenhoek's microscopes revealed an array of microbial life. For example, his descriptions of organisms found in lake water were the first observations of the green algae called Spirogyra and another microbe that came to be known as Vorticella. Finally, his observation of "animacules" in tooth plaque was the first visual detection of bacteria.