is 25, that of the calorific spectrum will be 42.10, and of the chemical
spectrum 55.10. Such a series of circles may well be used to represent
a beam from the sun, which may be regarded as an atom of Light,
surrounded with an invisible atmosphere of Heat, and another still
more extended, which possesses the remarkable property of producing
chemical and molecular change.

A ray of light, in passing obliquely through any medium of uniform density,
does not change its course; but if it should pass into a denser body,
it would turn from a straight line, pursue a less oblique direction,
and in a line nearer to a perpendicular to the surface of that body.
Water exerts a stronger refracting power than air; and if a ray of light
fall upon a body of this fluid its course is changed, as may be seen
by reference to Fig. 4.

[amdg_4.gif]

It is observed that it proceeds in a less oblique direction
(towards the dotted line), and, on passing on through, leaves the liquid,
proceeding in a line parallel to that at which it entered. It should be
observed that at the surface of bodies the refractive power is exerted,
and that the light proceeds in a straight line until leaving the body.
The refraction is more or less, and in all cases in proportion
as the rays fall more or less obliquely on the refracting surface.
It is this law of optics which has given rise to the lenses in our
camera tubes, by which means we are enabled to secure a well-delineated
representation of any object we choose to picture.

When a ray of light passes from one medium to another, and through that into
the first again, if the two refractions be equal, and in opposite directions,