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.
Lenses and Cylinders. With the exception of primitive eyeglasses, the telescope was the first optical instrument constructed, yet its origin is surrounded by controversy. The most likely story places its invention around 1600 in the shop of an obscure Dutch eyeglass maker named Hans Lippershey.