Transparent bodies, such as glass, may be made of such form as to
cause all the rays which pass through them from any given point to
meet in any other given point beyond them, or which will disperse them
from the given point. These are called lenses, and have different
names according to their form. 1. Is called the plano-convex lens. 2.
Plano-concave. 3. Double convex. 4. Double concave. 5. A meniscus, so
called from its resembling the crescent moon.