Page:The wonders of optics (1869).djvu/121

Rh this chapter. That is to say, the focus will approach more or less to the centre of curvature, according as the source of light is placed nearer to or further from the mirror; consequently, in the case of the candle in fig. 19, instead of uniting at , the rays will meet at f, a point situated somewhat nearer the mirror than the principal focus. If, instead of placing the light at, we place it at f, we shall find the rays will be concentrated at the point. Thus the foci are consequently related to each other, and are hence called conjugate foci. It will be readily seen that a spherical mirror may have an infinite number of conjugate foci, according to the 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.



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