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.
Then, around 1600, it was discovered that optical instruments could be made by combining lenses. The First Microscopes. The early simple microscopes which were only magnifying glasses had one power, usually about 6x - 10x. One thing that was very common and interesting to look at, were fleas and other tiny insects, hence these early magnifiers called flea glasses.