Thus we see the reason why certain rays produce certain effects on the eye, each particular degree of refraction causing a different set of vibrations, resulting in a different sensation for every part of the spectrum, and reproducing the effect of various colours on the optic nerve. In the following chapters we shall conclude our account of the different colours in the spectrum and of the laws of light. ------------------------------------------------------------------------ CHAPTER V. THE LAWS OF REFLECTION.—MIRRORS. WHEN a ray of light falls obliquely on any polished surface, as that of a mirror, a piece of water, a plate of burnished metal, or any other reflecting substance, the ray, like an elastic ball, is immediately projected in a contrary direction to that in which it fell. Moreover, the direction in which it is reflected is at right angles to the surface, and in the same plane as that of the ray in the first instance. This experiment may be tried very easily, and will show the reason for the two following laws. 1. The angle of incidence is equal to the angle of reflection, and _vice versâ_. 2. Reflection can only take place in one direction—in that of the incident rays, both of which are always in a plane perpendicular to the reflecting surface. The following figure will assist the student in performing experiments on the reflection of light from flat surfaces. The ray A B falling obliquely on the horizontal mirror, is reflected upwards at the same angle in the direction B C. This may be proved geometrically by placing a graduated circle in a vertical position in the plane A B C, when we shall find that the angle A B D formed by A B (the incident ray) with the perpendicular D B is equal to the angle formed by this perpendicular line and the reflecting ray B C. You may also prove in the same way that these three lines are all in the same vertical plane. [Illustration: FIG. 13.—Reflection from Plane Surfaces. ] Let us now examine the effects of light reflected from plane surfaces. We must first, however, notice a certain optical illusion to which we are continually falling a prey, almost without our knowledge. We always fancy objects to be in reality in the place where we see them, and, in spite of our having already enumerated a large number of these deceptions, we must still add one more to the list. In reality we rarely see objects in the place where they really are; for if by the effect of reflection, refraction, or any other cause, the rays of light are made to deviate from their course, we no longer see the object from which they proceed in its real position, but in the direction taken by the luminous pencil at the moment of entering the eye. [Illustration: FIG. 14.—Refraction. ] [Illustration: FIG. 15.—Experimental Proof of Refraction. ]