Towards the end of the sixteenth century and the early part of the seventeenth century, interest in the minute structure of natural objects appears to have developed. As early as 1590, Thomas Mouffet used magnifying glasses in studying small mites, and in 1637 Descartes invented a single lens microscope in which the rays of light were reflected on to the object by means of a concave mirror. This method of illumination, it is interesting to note, is still used in some forms of pocket magnifiers. Most of the early discoveries were made with single lenses, for in the compound microscopes which were first made, it was only possible to view such a small portion of an object at one time that the advantage lay with the less complicated instrument. The earliest microscopes were simply short tubes of any material which would not admit light; at one end there was a lens, at the other a glass plate on which the object to be examined was placed. Because these crude instruments were chiefly used for the examination of insects they were known as “Vitrea pulicaria” or “Vitrea muscaria.” Later they were called “Engyoscopes,” and, after the invention of compound microscopes, they were described as “Microscopia ludicra,” as opposed to the latter instruments, known as “Microscopia seria.” The next stage in the development of the microscope consisted in the introduction of lenses of very short focal length, and, in 1665, Robert Hooke used small glass balls, formed by fusing threads of drawn glass, for this purpose. It was Antony van Leeuwenhoek, however, who perfected these instruments. He brought an extraordinary skill and industry to bear on the grinding and polishing of minute lenses of short focal length. Already in 1673 Regnier de Graaf wrote to the Royal Society in London that Leeuwenhoek was making glasses far superior to those of the great Italian lens maker, Eustachio Divini. Leeuwenhoek’s success was largely due not only to his method of grinding, but also to the skill with which he mounted his lenses, which were accurately fitted into a minute hole in a metal plate. The object to be examined was firmly held in a stand and adjusted by means of a screw movement. By this means, and by the use of hollow metal reflectors, he succeeded in availing himself of transmitted light in the case of transparent objects. Leeuwenhoek was able to make immense advances with these instruments, the minute pond animals he could see with ease, and by 1683 he had even attained a sight of the bacteria. His researches represented the high-water mark of work done with the simple microscope, most of the later work was carried out with the compound instrument.