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_.
From this digression I pass on to the evolution of the compound microscope. The earliest workable form known was that designed by Eustachio Divini, who brought it to the notice of the Royal Society in 1668. It consisted of two plano-convex lenses, combined with their convex surfaces retained in apposition. His idea was subsequently improved upon by a London optician. Not long afterwards, Philip Bonnani published an account of his improved compound microscope; and we are certainly indebted to him for two or more forms of the movable horizontal microscopes, and for the compound condenser fitted with focussing gear for illuminating transparent objects by transmitted light. I must, however, pass by the many changes made in the structure and form of the instrument by the celebrated Dr. Culpeper, Scarlet, Cuff, and many other inventors.