w against the
cloud--the bow once said, when men transferred their own motives and
actions to the Divinity, to be the weapon of God?

[Sidenote: Optical discoveries.] The first ascertained optical fact was
probably the propagation of light in straight lines. The theory of
perspective, on which the Alexandrian mathematicians voluminously wrote,
implies as much; but agreeably to the early methods of philosophy, which
were inclined to make man the centre of all things, it was supposed that
rays are emitted from the eye and proceed outwardly, not that they come
from exterior objects and pass through the organ of vision inwardly.
Even the great geometer Euclid treated the subject on that erroneous
principle, an error corrected by the Arabians. In the meantime the law
of reflexion had been discovered; that for refraction foiled Alhazen,
and was reserved for a European. Among natural optical phenomena the
form of the rainbow was accounted for, notwithstanding a general belief
in its supernatural origin. Its colours, however, could not be explained
until exact ideas of refrangibility, dispersion, and the composition of
white light were attained. The reflecting telescope was invented; the
recognized possibility of achromatism led to an improvement in the
refractor. A little previously the progressive motion of light had been
proved, first for reflected light by the eclipses of Jupiter's
satellites, then for the direct light of the stars. A true theory of
colours originated with the formation of the solar spectrum; that
beautiful experiment led to the discovery of irrationality of dispersion
and the fixed lines. The phenomena of refraction in the case of Iceland
spar were examined, and the law for the ordinary and extraordinary rays
given. At the same time the polarization of light by double refraction
was discovered. A century later it was followed by polarisation by
reflexion and single refraction, depolarization, irised rings, bright
and black crosses in crystals, and unannealed or compressed glass, the
connexion between optical phenomena and crystalline form, uniaxial
crystals giving circular rings and biaxial oval ones, and circular and
elliptical polarization.

The beautiful colours of soap-bubbles, at first mixed up with those of
striated and dotted surfaces, were traced to their true
condition--thickness. The determination of thickness of a film necessary
to give a certain colour was the first instance of exceedingly min