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.
Crude telescopes and spyglasses may have been created much earlier by Dutch opticians, but Lippershey is believed to be the first to apply for a patent for his design. His telescope was made up of one convex lens and one concave lens. Galileo Galilei perfected it by using two convex lenses.