distance of the source of light. It is also clear, that if we cause the light to approach the mirror, the focus will also approach it. [Illustration: FIG. 19.—Conjugate Foci. ] Continuing our experiment, we shall find that when the candle passes the principal focus so as to be between it and the mirror, the reflected rays first become parallel and then divergent, and cannot consequently produce any focus beyond the mirror, but are reflected in the way shown in fig. 20. In experimenting on the plane mirror, we imagined we saw the object at a certain distance behind it; the same thing happens when we see ourselves reflected in a concave mirror, and the particular point at which we suppose we see our reflection is called the virtual focus. [Illustration: FIG. 20.—Virtual Focus. ] If instead of a candle we place our head before a concave mirror, we shall see ourselves magnified as in fig. 21. [Illustration: Fig. 21.—Concave Mirror. ] We shall easily see how this happens by tracing the paths of the rays in fig. 22. [Illustration: Fig. 22.—Magnifying effect of Concave Mirrors. ] [Illustration: Fig. 23.—The Reversal of real Images. ] The rays, for instance, which proceed from the forehead at the point _a_ are reflected from the point _o_ to the eye in such a way as to appear to proceed from a point beyond the mirror, A. In the same manner the rays reflected from the chin appear to take their origin from the point B. If, on the other hand, we place ourselves at a distance from the principal focus, we shall produce a reversed and diminished image of our face. This image is not illusory, like the preceding ones, but is real, and may be received upon a screen, as shown in fig. 23. We may easily follow the path of the rays as shown in the figure, and we shall see that the rays forming the images of the church-tower and the terrace below, cross at a certain point. Convex mirrors produce precisely opposite effects, and give a diminished image instead of a magnified one, as may be perceived on examining fig. 24. [Illustration: FIG. 24.—Diminishing power of Convex Mirrors. ] ------------------------------------------------------------------------ CHAPTER VI. METALLIC BURNING MIRRORS.