Answer: 20 years. Furnaces should last between 15 and 20 years, according to industry experts and the Department of Energy. Furnace filters, however, only hold up one to three months â and in order to avoid an expensive service call, it's crucial to change them on the manufacturer's schedule.
Lens Improvement. Later in the 16th century, Anton van Leeuwenhoek began polishing and grinding lenses when he discovered that certain shaped lenses increased an imageâs size. The glass lenses that he created could enlarge an object many times.
Evidence points to the first compound microscope appearing in the Netherlands in the late 1590s, probably an invention of eyeglass makers there, Hans Lippershey (who developed an early telescope) and Zacharias Janssen (also claimed as the inventor of the telescope).