REFLEXIONS IN SMOOTH WATER 7 mage of P. If the eye is moved to any other point, such as H, the image still occupies the same position I; in other words, all rays from P falling upon the mirror appear after reflexion to proceed from I, the image of P. In the same way, if in Fig. 1 the lines DC, GF and KH be produced backwards, they will be found all to meet ina point I, and if a straight line be drawn from P to I, this line will be at right angles to, and bisected by, the surface of the mirror at the point O.' So that to find the image in a plane mirror of any point we have the following rule: draw a perpendicular from that point to the surface? of the mirror, and produce tt until its length is doubled. Thus it is evident that I is fixed relatively to P, and as long as P and the mirror are stationary, I always occupies the same position, whatever the position of the observer. It follows that there can be only one image of a point in a plane reflecting surface; when we come to consider reflexions in rippled water, however, we shall find that this is not the case and that a single point may have a great number of images. Having got the image of a single point, it is a simple matter to construct the image of a solid object. If we hold any object, as for instance a candlestick, ‘ The proofs of the elementary propositions of Optics made use of in this chapter, which follow from the law of reflexion given above, may be found in any text book of Physics. * The surface of the mirror must be produced, if necessary, to meet the perpendicular. In the case of water, we may say, “ draw a perpendicular from the point in question to the /ve/ of the water and produce it until its length is doubled” ; for of course objects are visible by reflexion that are far beyond the actual extent of the water.