ute
measures beautifully executed. These soon became connected with fringes
in shadows, and led to ascertaining the length of waves of light.

[Sidenote: Vision; the functions of the eye.] Meantime more correct
ideas respecting vision were obtained. Alhazen's explanation of the use
of the retina and lens was adopted. This had been the first truly
scientific investigation in physiology. The action of the eye was
reduced to that of the camera-obscura described by Da Vinci, and the old
notion of rays issuing therefrom finally abandoned. It had held its
ground through the deceptive illustration of the magic-lantern. Of this
instrument the name indicates the popular opinion of its nature. In the
stories of necromancers and magicians of the time are to be found traces
of applications to which it was insidiously devoted--the raising of the
dead, spectres skipping along the ground or dancing on the walls and
chimneys, pendulous images, apparitions in volumes of smoke. [Sidenote:
Optical instruments.] These early instruments were the forerunners of
many beautiful inventions of later times--the kaleidoscope, producing
its forms of marvellous symmetry: the stereoscope, aided by photography,
offering the very embodiment of external scenery; the achromatic and
reflecting telescope, to which physical astronomy is so greatly
indebted; and the achromatic microscope, now working a revolution in
anatomy and physiology.

[Sidenote: The undulatory theory.] In its theory optics has presented a
striking contrast to acoustics. Almost from the very beginning it was
recognized that sound is not a material substance emitted from the
sounding body, but only undulations occurring in the air. For long,
optics failed to reach an analogous conclusion. The advancement of the
former science has been from the general principle down to the details,
that of the latter from the details up to the general principle.

That light consists of undulations in an elastic medium was first
inferred in 1664. Soon after, reflexion, refraction, and double
refraction were accounted for on that principle. The slow progress of
this theory was doubtless owing to Newton's supremacy. He gave a
demonstration in the second book of the "Principia" (Prop. 42) that wave
motions must diverge into the unmoved spaces, and carried popular
comprehension with him by such illustrations as that we hear sounds
though a mountain interpose. It was thought that the undulatory theory
was d