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.
There are other claims that the microscope and the telescope were invented by Roger Bacon in the 13th century, but this is not substantiated. Giovanni Faber coined the name microscope for Galileo Galilei 's compound microscope in 1625 (Galileo had called it the  occhiolino  or  little eye ).