by the other. You obtain a certain quotient. In like manner divide
_m'_ _o'_ by the corresponding perpendicular _p'_ _n'_; you obtain
precisely the same quotient. Snell, in fact, found this quotient to be
_a constant quantity_ for each particular substance, though it varied
in amount from one substance to another. He called the quotient the
_index of refraction_.

[Illustration Fig. 5]

In all cases where the light is incident from air upon the surface of
a solid or a liquid, or, to speak more generally, when the incidence
is from a less highly refracting to a more highly refracting medium,
the reflection is _partial_. In this case the most powerfully
reflecting substances either transmit or absorb a portion of the
incident light. At a perpendicular incidence water reflects only 18
rays out of every 1,000; glass reflects only 25 rays, while mercury
reflects 666 When the rays strike the surface obliquely the reflection
is augmented. At an incidence of 40°, for example, water reflects 22
rays, at 60° it reflects 65 rays, at 80° 333 rays; while at an
incidence of 89½°, where the light almost grazes the surface, it
reflects 721 rays out of every 1,000. Thus, as the obliquity
increases, the reflection from water approaches, and finally quite
overtakes, the perpendicular reflection from mercury; but at no
incidence, however great, when the incidence is from air, is the
reflection from water, mercury, or any other substance, _total_.

Still, total reflection may occur, and with a view to understanding
its subsequent application in the Nicol's prism, it is necessary to
state when it occurs. This leads me to the enunciation of a principle
which underlies all optical phenomena--the principle of