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.
Because at any given point on Earth the weight of an object is proportional to its mass, the mass of an object in kilograms is usually measured by comparing its weight to the weight of a standard mass, whose mass is known in kilograms, using a device called a weighing scale.