Optics is the science of _light_ and _vision_. Concerning the nature
of light, two theories are at present very ably maintained by their
respective advocates. One is termed the Newtonian theory, and the
other the Huygenean. The Newtonian theory considers light to consist
of inconceivably small bodies emanating from the sun, or any other
luminous body. The Huygenean conceives it to consist in the undulations
of a highly elastic and subtle fluid, propagated round luminous centers
in spherical waves, like those arising in a placid lake when a stone is
dropped into the water.