As early as 1673 Leeuwenhoek communicated some important discoveries made by a simple microscope of his own construction to the Royal Society; he, however, gave no particulars of the construction of the instrument. Dr. Adams, writing to his friend (Sir) Hans Sloane, says: “They appear to be spherules lodged between two plates of gold or brass, in a hole whose diameter appears to be no bigger than that of a small pin’s head.” At his death he bequeathed to the Royal Society a cabinet containing twenty-six of these microscopes; the cabinet and the microscopes long ago disappeared, but not before they were carefully examined and described by Mr. Henry Baker, F.R.S. In his report to the Royal Society, he says: “They consisted of a series of convex-lenses, ranging in power from 1·20 to 1·5, and magnifying from 160 to 40 diameters.” This must now be regarded as an eventful period in the history of the microscope, since Leeuwenhoek’s discoveries created a great sensation throughout Europe. And all further improvements in compound instruments appear to have been laid aside for some considerable period in consequence: and the pocket instrument of Wilson, together with that of his scroll standard (seen on the cover of this book), and which was one of the first simple microscopes with a mirror mounted on the base in a line with the optic axis. The discoveries once more made, and at a much later period (1738), by Dr. Nathaniel Lieberkuhn with his simple microscopes, and by means of which he discovered the minute structure of the mucous membrane of the alimentary canal, and which alone would have immortalised his name had we not preserved in use to this day an important adjunct of every modern instrument, the Lieberkuhn reflector. In the Museum of the Royal College of Surgeons of England, there is a small cabinet of two drawers, containing a set of twelve of his simple microscopes, each being provided with an original injection. The form of the instrument is shown in Figs. 1 and 2. _a b_ represents a piece of brass tubing about an inch long and an inch in diameter and provided with a cap at each extremity. The one at _a_ carries a small double-convex lens of half an inch focal length; while at _b_ there is fixed a condensing lens three-quarters of an inch in diameter. In Fig. 2 the instrument is seen in section, and explains itself. It is held by the handle in such a position that the rays of light, from a lamp or a white cloud, may fall on the condenser _b_, and concentrate on the speculum _l_. This again further condenses the rays on the disc _c_, where the object is held, and its adjustment made by the milled-head screw _d_, so as to bring it within the focus of the lens _a_. [Illustration: Fig. 1.]