Microscopy, Optical

Hooke and Leeuwenhoek

People have peered through quartz or glass to magnify objects since antiquity. Like alchemy, astronomy, algebra, and much of medicine, classical knowledge of lenses was preserved in the Arab and Muslim world. It was only in the 14th century that Western Europeans figured out how to grind lenses for spectacles. Two centuries later, Dutch lensmakers had learned to combine multiple lenses, thereby inventing the telescope and the microscope.

Italian natural philosophers, including Galileo, took up microscopy in the 1620s, but elsewhere it was not an important tool of the Scientific Revolution until the 1660s. At that point, alchemists and philosophers of matter such as Robert Boyle came to believe that the microscope could provide direct evidence for “corpuscles,” the particulate constituents of matter that—perhaps through differences in shape—accounted for observed qualities such as the taste and hardness of materials. Boyle's assistant, Robert Hooke, published a beautifully illustrated compendium of microscope images, the Micrographia of 1665. Boyle also encouraged an extraordinarily gifted Dutch lensmaker, Antoni van Leeu-wenhoek, whose keen eyesight and exquisite simple microscopes offered resolution of the microscale that was unsurpassed until the 19th century.

Abbe and Aberrations

By 1700, though, it was obvious that microscopes would be unable to resolve the elementary constituents of matter, and natural philosophers rapidly lost interest. Microscopes contributed little to science for the next century. However, technical improvements starting in the 1820s—particularly new lenses to correct for spherical and chromatic aberration, and new dyes to stain biological samples—reignited interest, particularly from botanists and zoologists.

In the 1870s, Ernst Abbe, a physicist working for the Zeiss workshop of instrument makers, developed a theoretical understanding of optical microscopy that has defined the technique ever since. At the time, Abbe's work was probably meant simply to advertise Zeiss' microscopes as having a scientific underpinning. Over the medium term, Abbe guided Zeiss to more powerful instruments. In the long run, though, Abbe's work established a limit to improvements in optical microscopy. No matter how many aberrations microscope designers overcame, the smallest object a light microscope could resolve would be directly proportional to the wavelength of the light shining on the object, and inversely proportional to the sine of the index of refraction of the material from which the microscope's lenses were made.

Abbe's limit implies that even the best optical microscope will be unable to distinguish features less than about 200 nanometers apart. This would seem to bar optical microscopy from use in nanoscience. It also implies a limit to the use of optical lithography (the inverse of microscopy) in microelectronics manufacturing. Yet ingenious design gradually overcame Abbe's limit in both microscopy and lithography.

...