The early history of telescopes shows that the effect of combining two lenses was understood by scientists long before any particular use was made of this knowledge; and that those who are accredited with introducing perspective glasses to the public hit by accident upon the invention. Priority was claimed by two firms of spectacle-makers in Middelburg, Holland, namely, Zacharias, miscalled Jansen, and Lippershey. Galileo heard of the contrivance in July, 1609, and soon furnished so powerful an instrument of discovery that things seen through it appeared more than thirty times nearer and almost a thousand times larger than when seen by the naked eye. He was able to make out the mountains in the moon, the satellites of Jupiter in rotation, the spots on the revolving sun; but his telescope afforded only an imperfect view of Saturn. Of course these facts, published in 1610 (_Sidereus Nuncius_), strengthened his advocacy of the Copernican system. Galileo laughingly wrote Kepler that the professors of philosophy were afraid to look through his telescope lest they should fall into heresy. The German astronomer, who had years before written on the optics of astronomy, now (1611) produced his _Dioptrice_, the first satisfactory statement of the theory of the telescope. About 1639 Gascoigne, a young Englishman, invented the micrometer, which enables an observer to adjust a telescope with very great precision. Before the invention of the micrometer exactitude was impossible, because the adjustment of the instrument depended on the discrimination of the naked eye. The micrometer was a further advance in exact measurement. Gascoigne's determinations of, for example, the diameter of the sun, bear comparison with the findings of even recent astronomical science. The history of the microscope is closely connected with that of the telescope. In the first half of the seventeenth century the simple microscope came into use. It was developed from the convex lens, which, as we have seen in a previous chapter, had been known for centuries, if not from remote antiquity. With the simple microscope Leeuwenhoek before 1673 had studied the structure of minute animal organisms and ten years later had even obtained sight of bacteria. Very early in the same century Zacharias had presented Prince Maurice, the commander of the Dutch forces, and the Archduke Albert, governor of Holland, with compound microscopes. Kircher (1601-1680) made use of an instrument that represented microscopic forms as one thousand times larger than their actual size, and by means of the compound microscope Malpighi was able in 1661 to see blood flowing from the minute arteries to the minute veins on the lung and on the distended bladder of the live frog. The Italian microscopist thus, among his many achievements, verified by observation what Harvey in 1628 had argued must take place.