another, it is impossible so to understand what I have been saying
about two persons mutually seeing one another's eyes, or how two
torches can illuminate one another.




CHAPTER II

ON REFLEXION


Having explained the effects of waves of light which spread in a
homogeneous matter, we will examine next that which happens to them on
encountering other bodies. We will first make evident how the
Reflexion of light is explained by these same waves, and why it
preserves equality of angles.

Let there be a surface AB; plane and polished, of some metal, glass,
or other body, which at first I will consider as perfectly uniform
(reserving to myself to deal at the end of this demonstration with the
inequalities from which it cannot be exempt), and let a line AC,
inclined to AD, represent a portion of a wave of light, the centre of
which is so distant that this portion AC may be considered as a
straight line; for I consider all this as in one plane, imagining to
myself that the plane in which this figure is, cuts the sphere of the
wave through its centre and intersects the plane AB at right angles.
This explanation will suffice once for all.

[Illustration]