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.
Before the Janssen microscope, magnification was limited, accomplished with a single lens. Zacharias and his father used a simple tube but inserted lenses at both ends. One was convex; the other, concave. William Boreel described the tube as over 2 feet long but no wider than 2 inches in diameter. According to his report, the instrument rested on a tripod that resembled dolphins.