instance, if a candle be placed as shown, and a screen on the other side of the lens, a point will be found where the image of the candle is seen upon it in a reversed position. The distance between these two points is always relative, and they are called conjugate foci. Thus, the candle may change places with the screen with a similar effect, as long as the exact position of the two points is preserved. If the candle is placed
Fig. 30.—Conjugate Foci.
farther off, we must diminish the distance between the screen and the lens, and vice versâ. In fact, the nearer the object, the longer the focus; the farther it is off, the shorter the focus. Half an hour's experiment with a double convex lens, a piece of white card-board, and a small candle, will teach the student more about the properties of convex lenses than a chapter of explanation. A common magnifying glass, or even an old spectacle lens, will serve the purpose of more expensive instruments.
We now proceed to speak of the images formed by lenses. In fig. 31 we have a flower placed on one side of a lens. As it is not at an infinite distance, the rays sent out by its various parts are convergent, and not parallel, consequently they do not meet at the sidereal focus, but at a point beyond it, according to the rule already laid down. The rays proceeding from the exact centre of the flower striking the lens exactly in
