Scleral lenses have gained popularity in the last decade. From a lens with limited viability for over 100 years to the fastest growing segment of the gas permeable lens market, scleral lenses have come a long way.
Mass and Weight. The mass of an object is a fundamental property of the object; a numerical measure of its inertia; a fundamental measure of the amount of matter in the object. Definitions of mass often seem circular because it is such a fundamental quantity that it is hard to define in terms of something else.
A measure of how much matter is in an object. This gold bar is quite small but has a mass of 1 kilogram (about 2.2 pounds), so it contains a lot of matter. Mass is commonly measured by how much something weighs. But weight can change for different locations (such as on the moon) while the mass stays the same. Mass is measured in grams, kilograms and, tonnes (Metric) or ounces and pounds (US units) See: Weight