The power of the microscope as an instrument of biological research was in some measure revealed by Hooke's _Micrographia_ (1665). Robert Hooke was a man of extraordinary ingenuity and scientific fertility, who took a leading part in the early work of the Royal Society. He opens his book with an account of the simple and the compound microscope of his own day, and then goes on to explain, with the help of large and elaborate engraved plates, the structure of a number of minute objects. The most interesting are: A Foraminiferous shell, snow-crystals, a thin section of cork showing its component cells, moulds, a bit of Flustra, the under side of a nettle-leaf with its epidermic cells and stinging-hairs, the structure of a feather, the foot of a fly, the scales of a moth's wing, the eye of a fly, a gnat-larva, and a flea. The beauty of the plates and the acuteness of some of the explanations are remarkable, but lack of connection between the topics discussed hinders the _Micrographia_ from rising to a very high scientific level. Swammerdam treated the microscope as an instrument of continuous biological research. In his eyes it was a sacred duty to explore with the utmost faithfulness the minute works of the Creator. Insects yielded him an inexhaustible supply of natural contrivances, in which closer scrutiny always brought to view still more exquisite adaptations to the conditions of life. He was able to throw a beam of steady light upon the perplexed question of insect-transformation, and swept from his path the sophistries with which the philosophy of the schools had obscured the change of the caterpillar into a moth, or of the tadpole into a frog. He demonstrated the gradual progress of the apparently sudden transformation of certain insects by dipping into boiling water a full-fed caterpillar, and then exposing the parts of the moth or butterfly, which had almost attained their complete form beneath the larval skin; after this it was easy to discover the same parts in the pupa.