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.
Around 1600, the microscope was invented, possibly by Hans and Zacharias Jansen. Due to poor lens quality, the early compound microscopes (ones that used two lenses) could only magnify an object up to 20 or 30 times its normal size.