Hooke, at one time research assistant to Robert Boyle, was one of the greatest inventive geniuses of all time. This was the first book devoted entirely to microscopical observations, and also the first book to pair its microscopic descriptions with profuse and detailed illustrations. The 38 copperplate engravings in the book were mostly after drawings by Hooke; some were probably after drawings by the architect and occasional scientist, Sir Christopher Wren. This graphic portrayal of the hitherto unknown microcosm had an impact rivalling that of Galileo's Sidereus nuncius (1610), which was the first book to include images of the macrocosm shown through the telescope. It was also the second book published under the auspices of the Royal Society of London.

Hooke constructed one of the most famous of the early compound microscopes, and began his observations with studies of non-living materials, such as woven cloth and frozen urine crystals, then proceeded to investigations of plant and animal life. He published the first studies of insect anatomy, giving a lucid account of the compound eye of the fly, and illustrating the microscopic details of such structures as apian wings, flies' legs and feet, and the sting of the bee. His famous and dramatic portraits of the flea and louse, a frightening eighteen inches long, are hardly less startling today than they must have been to Hooke's contemporaries. His botanical observations include the first description of the plant-like form of molds, and of the honeycomb-like structure of cork, which last he described as being composed of "cellulae"— thereby coining the modern biological usage of the work "cell" to describe the basic microscopic units of tissue. Digital facsimile from the National Library of Medicine at this link.

By blowing air from a bellows over the exposed lungs of a dog, Hooke proved that respiratory motion is not necessary to maintain life, but that the essential feature of respiration lies in certain blood changes in the lungs. Reprinted in J. F. Fulton’s Selected readings in the history of physiology, 2nded., 1966, pp. 121-23.

On his title page Hooke listed the contents of this section as follows:

"Mr. Leeuwenhoeck's two letters concerning some late microscopical discoveries.
"The author's discourse and description of microscopes, improved for discerning the nature and texture of bodies.
"P. Cherubine's accusations answered.
"Mr. Young's letter containing several anatomical observations."

Digital facsimile from Google Books at this link.

Reprints the auction catalogue of Hooke's library: Bibliotheca Hookiana (1703).