That, for the moment at least, exhausted the interest in the Heavens, and man began to study the earth. The invention of a workable microscope, (a strange and clumsy little thing,) by Anthony van Leeuwenhoek during the last half of the 17th century, gave man a chance to study the "microscopic" creatures who are responsible for so many of his ailments. It laid the foundations of the science of "bacteriology" which in the last forty years has delivered the world from a great number of diseases by discovering the tiny organisms which cause the complaint. It also allowed the geologists to make a more careful study of different rocks and of the fossils (the petrified prehistoric plants) which they found deep below the surface of the earth. These investigations convinced them that the earth must be a great deal older than was stated in the book of Genesis and in the year 1830, Sir Charles Lyell published his "Principles of Geology" which denied the story of creation as related in the Bible and gave a far more wonderful description of slow growth and gradual development.