figure 15.
Mr. Walker illustrates the manner in which we see our faces in a mirror by the following figure (16). AB represents a mirror, and OC, a person looking into it. If we conceive a ray proceeding from the forehead CE, it will be sent to the eye at O, agreeably to the angle of incidence and reflection. But the mind puts CEO into one line, and the forehead is seen at H, as if the lines CEO had turned on a hinge at E.—It seems a wonderful faculty of the mind to put the two oblique lines CE and OE into one straight line OH, yet it is seen every time we look at a mirror. For the ray has really travelled from C to E, and from E to O, and it is that journey which determines the distance of the object; and hence we see ourselves as far beyond the mirror as we stand from it. Though a ray is here taken only from one part of the face, it may be easily conceived that rays from every other part of the face must produce a similar effect.
figure 16.
In every plain mirror, the image is always equal to the object, at what distance soever it may be placed; and as the mirror is only at half the distance of the image from the eye, it will completely receive an image of twice its own length. Hence a man six feet high may view himself completely in a looking glass of three feet in length, and half his own breadth; and this will be the case at whatever distance he may stand from the glass. Thus, the man AC (fig. 17) will see the whole of his own image in the glass AB, which is but one half as large as himself. The rays from the head pass to the mirror in the line Aa, perpendicular to the mirror, and are returned to the eye in the same line; consequently, having travelled twice the length Aa, the man must see his head at B. From his feet C rays will be sent to the bottom of the mirror at B; these will be reflected at an equal angle to the eye in the direction BA, as if they had proceeded in the direction DbA, so that the man will see his foot at D, and consequently his whole figure at BD.
figure 17.
A person when looking into a mirror, will always see his own image as far beyond the mirror as he is before it, and as he moves to or from it, the image will, at the same time, move towards or from him on the other side; but apparently with a double velocity, because the two motions are equal and contrary. In like manner, if while the spectator is at rest, an object be in motion, its image behind the mirror will be seen to move at the same time. And if the spectator moves, the images of objects that are at rest will appear to approach, or recede from him, after the same manner as when he moves towards real objects; plane mirrors reflecting not only the object, but the distance also, and that exactly in its natural dimensions—The following principle is sufficient for explaining most of the phenomena seen in a plane mirror, namely;—That the image of an object seen in a plane mirror, is always in a perpendicular to the mirror joining the object and the image, and that the image is as much on one side the mirror, as the object is on the other.