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.
Mass is a measure of the matter within a body. Mass is measured in kilograms and grams (metric) and slugs (English). Weight is the force of gravitational attraction on a body as observed on the rotating Earth or similar situation. Weight on Earth and weight on the Moon of the same object, is different.