_Mrs. B._ Your question leads to a very important property of light, _Reflection_. When rays of light encounter an opaque body, they cannot pass through it, and part of them are absorbed by it, and part are reflected, and rebound; just as an elastic ball rebounds, when struck against a wall. By reflection, we mean that the light is turned back again, through the same medium which it had traversed in its first course. _Emily._ And is light, in its reflection, governed by the same laws, as solid, elastic bodies? _Mrs. B._ Exactly. If a ray of light fall perpendicularly on an opaque body, it is reflected back in the same line, towards the point whence it proceeded. If it fall obliquely, it is reflected obliquely, but in the opposite direction; the ray which falls upon the reflecting surface, is called the incident ray, and that which leaves it, the reflected ray; the angle of incidence, is always equal to the angle of reflection. You recollect that law in mechanics? _Emily._ Oh yes, perfectly. _Mrs. B._ If you will shut the shutters, we will admit a ray of the sun's light, through a very small aperture, and I can show you how it is reflected. I now hold this mirror, so that the ray shall fall perpendicularly upon it. _Caroline._ I see the ray which falls upon the mirror, but not that which is reflected by it. _Mrs. B._ Because it is turned directly back again; and the ray of incidence, and that of reflection, are confounded together, both being in the same line, though in opposite directions. _Emily._ The ray then, which appears to us single, is really double, and is composed of the incident ray, proceeding to the mirror, and of the reflected ray, returning from the mirror. _Mrs. B._ Exactly so. We will now separate them, by holding the mirror M, (fig. 6,) in such a manner, that the incident ray, A B, shall fall obliquely upon it--you see the reflected ray, B C, is marching off in another direction. If we draw a line from the point of incidence B, perpendicularly, to the mirror, it will divide the angle of incidence, from the angle of reflection, and you will see that they are equal. _Emily._ Exactly; and now, that you hold the mirror, so that the ray falls more obliquely upon it, it is also reflected more obliquely, preserving the equality of the angles of incidence, and of reflection. _Mrs. B._ It is by reflected rays only, that we see opaque objects. Luminous bodies, send rays of light immediately to our eyes, but the rays which they send to other bodies, are invisible to us, and are seen, only when they are reflected by those bodies, to our eyes. _Emily._ But have we not just seen the ray of light, in its passage from the sun to the mirror, and its reflections? yet, in neither case, were those rays in a direction to enter our eyes. _Mrs. B._ What you saw, was the light reflected to your eyes, by small particles of dust floating in the air, and on which the ray shone, in its passage to, and from, the mirror.