n its movements by various conditions. In a substance
of uniform density it moves with an unchanging velocity and in a straight
line. If it enters a less dense, or rarer, substance, its velocity
increases; if one more dense, its velocity diminishes; and if it enters
either the rarer or denser substance in any direction other than
perpendicularly, it is bent out of its course, or _refracted_. If it
strikes against a body lying in its course, it may be thrown off
(_reflected_), or it may enter the body and either be passed on through
(_transmitted_) or _absorbed_ (Fig. 157). Light which is absorbed is
transformed into heat.

*Kinds of Reflection.*--Waves of light striking against the smooth surface
of a mirror are thrown off in definite directions, depending on the angle
at which they strike. (Illustrate by holding a mirror in the direct rays
of the sun.) But light waves that strike rough surfaces are reflected in
practically all directions and apparently without reference to the angle
at which they strike. (Illustrate by placing a piece of white paper in the
direct rays of the sun. It matters not from what direction it is viewed,
waves of light strike the eye.) This kind of reflection is called
_diffusion_, and it serves the important purpose of making objects
visible. The light waves passing out in all directions from objects which
have received light from the sun, or some other luminous body, enable them
to be seen.

[Fig. 157]

Fig. 157--*Diagram illustrating passage of light waves.*On the right the
light is transmitted by the glass, reflected by the mirror, refracted by
the prism, and absorbed by the black cloth. On the left the light from the
candle forms an image by passing through a small hole in a cardboard and
falling upon a screen.

*Formation of Images.*--Another principle necessary to seeing is that of
refraction. _Refraction_ means the bending, or turning, of light from a
straight course. One of the most interesting effects of refraction is the
formation of images of objects, such as may be accomplished by light from
them passing in a certain manner through convex lenses. If, for example, a
convex lens be moved back and forth between a candle and a screen in a
dimly lighted room, a position will be found where a picture of the candle
falls upon the screen. This picture, called the _image_, results from the
refraction of the candle light in p