Measuring Refraction.
The degree in which a crystal, or a particular kind of glass, bends a beam of light is usually measured by giving the crystal or glass the form of a prism, through which rays are sent. Sometimes a crystal is so small and irregular that this method is not feasible. Then the inquirer resorts to an indirect plan. He immerses the crystal in liquids which he mixes until the crystal disappears through ceasing to bend light differently from the surrounding bath. He then fills a hollow glass prism with this liquid, and in noting its refraction he learns that of the immersed crystal.
Blenkinsop’s locomotive, 1811.
Middleton Colliery, near Leeds, England.