In 1861, Pasteur published his germ theory [3]. The “Germ theory states that specific microscopic organisms are the cause of specific diseases…. The theory was developed, proved, and popularized in Europe and North America between about 1850 and 1920″, following closely the manufacture of fine optical microcopes from early 1850‘s. Microorganisms had already been discovered.
The Spencer [1, 2] microscopes were manufactured in the United States.
The first compound microscope (and telescope) is credited to Zacharias Janssen, the son of Hans Janssen, a spectacle maker from Middleburg, Holland, in the 1590s when Zach would have been a teenager. At this time eyeglasses were beginning to be popular. Though poor in image quality is a seminal advance in scientific instrumentation and discoveries. [3]