Page:Elementary Text-book of Physics (Anthony, 1897).djvu/408



322. Vision and light.—The ancient philosophers before Aristotle believed that vision consisted in the contact of some subtle emanation from the eye with the object seen. Aristotle showed the absurdity of this view by suggesting that if it were true, one should be able to see in the dark. Since his time it has been generally admitted that vision results from something proceeding from the body seen to the eye, and there impressing the optic nerve. This we call light.

Optics treats of the phenomena of light. It is conveniently divided into two branches: Physical Optics, which treats of the phenomena resulting from the propagation of light through space and through different media; and Physiological Optics, which treats of the sense of vision.

'''323. Theories of Light. The Ether.'''—The principal facts known about light in Newton's time, especially its propagation in straight lines, its reflection and refraction, could be explained by the hypothesis that light consisted of small material particles or corpuscles emitted from luminous bodies with very high velocities. This emission theory was adopted and defended by Newton.

Newton's contemporary Huygens proposed to explain the phenomena of light as the result of waves set up in luminous bodies and transmitted by an elastic medium which pervades all space. The properties which Huygens assigned to this medium were those