PHYSICAL SCIENCE IN SEVENTEENTH CENTURY 267 



Jansen, and Lippershey. Galileo heard of the contrivance in July 

 1609 and soon furnished so powerful an instrument of discovery that 

 ... he was able to make out the mountains in the moon, the satel- 

 lites of Jupiter in rotation, the spots on the revolving sun . . . 



About 1639, Gascoigne, a young Englishman, invented the mi- 

 crometer which enables an observer to adjust a telescope with very 

 great precision. 



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 

 . . . Leeuwenhoek before 1673 had studied the structure of minute 

 animal organisms and ten years later had even obtained sight of bac- 

 teria. Very early in the same century Zacharias had presented 

 Prince Maurice, the commander of the Dutch forces, and the Arch- 

 duke Albert, Governor of Holland, with compound microscopes. 

 Kircher (1601-1680) made use of an instrument that represented 

 microscopic forms at one thousand times larger than their actual 

 size. LIBBY. Introduction to the History of Science. 



The name of Galileo goes also with the invention of the ther- 

 mometer, an air, or more strictly a water, thermometer having 

 been introduced by him about 1597. Mercury was not substituted 

 for water until 1670, but alcohol thermometers, also introduced by 

 Galileo, were used much earlier. The freezing and boiling of 

 water were supposed to take place at variable temperatures and 

 it was not until the end of the seventeenth century that it was 

 realized that the freezing and the boiling points are invariable. 

 (For Galileo's other work in physics, see pp. 246-250.) Pendulum 

 clocks, " aerial " telescopes and the achromatic eye-pieces which 

 bear his name were introduced by Huygens, the first in 1657 

 and the others about 1680. 



The invention of the barometer by Torricelli has already been 

 described above (p. 257). The air-pump, though merely the appli- 

 cation of an ordinary pump to air instead of water, was so rich 

 in its results that it deserves a high place among the other and 

 more important inventions of this remarkable scientific era. 



About the origin of the (compound) microscope there is much 



