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.
JSS Round 2 Teacher Support Material Microscope History BC Egyptians used clear crystals to magnify small objects. 1st century AD Romans used magnifying glass. 13th century Eye spectacles were being used. The term lens was introduced (after lentils, the seeds). 16th century Jansen used compound lenses. 17th century Galileo improved focusing device. Van Leeuwenhoek perfected technique with lenses of great curvature, accepted as inventor of microscope. He described bacteria, yeast, blood cells. 19th century Achromatic lenses were first used.