The mid-17th century marked a profound shift in scientific inquiry, driven by the advent of magnifying lenses that opened up a previously invisible world. Robert Hooke, a polymath and Curator of Experiments for the newly formed Royal Society of London, stood at the forefront of this revolution. His work involved meticulously observing and documenting the minute details of nature and everyday objects, sharing the excitement of early microscopy with the scientific community. The observations he made of a commonplace material—a slice of tree bark—would permanently change the understanding of life’s fundamental structure.
Identifying the Instrument
The instrument Hooke used to examine the tree bark, specifically cork, was a Compound Microscope that he had a hand in designing and improving. This device was significantly more advanced than the simple microscopes of the time, which used only a single lens for magnification. A compound microscope employs at least two lens systems: an objective lens near the specimen and an eyepiece lens near the observer’s eye, allowing for a much greater degree of magnification. Hooke’s instrument, which was likely constructed by the London instrument maker Christopher Cock, was based on designs Hooke detailed and refined himself. The use of multiple lenses, while introducing optical challenges like chromatic aberration, provided the power necessary to visualize structures at a higher resolution than previously possible. His adoption of this technology was a clear statement about the direction of scientific discovery, moving toward more powerful, though optically complex, tools.
The Design and Functionality
Hooke’s compound microscope was a sophisticated piece of equipment for the era, constructed of materials like wood, brass, and pasteboard. The magnification was achieved through a combination of an objective lens, a field lens, and an eyepiece lens. For more accurate viewing of small parts, however, Hooke often removed the middle field lens, recognizing that fewer lens surfaces created a brighter and clearer image, even if the field of view was smaller. Perhaps the most inventive aspect of his apparatus was the illumination system, which was necessary because the compound lens arrangement often produced dark images. Hooke devised a system where light from a powerful oil lamp was focused onto the opaque specimen using a large, water-filled glass flask, which acted as a condenser.
The Discovery and Legacy
When Hooke examined a thin slice of cork under this microscope, he observed a multitude of tiny, regular compartments that were separated by rigid walls. These porous, box-like structures were not present in the living tissue but were the remains of the dead plant cells, primarily their cell walls. He famously coined the term “cell” because the small, empty spaces reminded him of the plain, small rooms, or cella, occupied by monks in a monastery. Hooke’s meticulous observations were published in 1665 in his seminal work, Micrographia: or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses. This book was a sensation, featuring detailed drawings and descriptions of his microscopic world, from the eye of a fly to the structure of a feather, and established the foundation for the field of cell biology.