at O, which consequently sees the image in the lines of reflection O D, O Q, and in the angle D O Q, and consequently magnified much beyond the natural size, and at a small distance behind the mirror. [Illustration: _figure 23._] [Illustration: _figure 24._] If we suppose the side T U to represent a _convex_ mirror, and the figure D Q a head of an ordinary size, then the figure A will represent the diminished appearance which a person’s face exhibits, when viewed in such a mirror. It will not only appear reduced, but somewhat distorted; because from the form of the mirror, one part of the object is nearer to it than another, and consequently will be reflected under a different angle. The effect we have now mentioned as produced by _concave_ mirrors, will only take place when the eye is nearer the mirror than its principal focus. If the spectator retire beyond this focus--suppose to the distance of five or six feet, he will not see the image _behind_ the mirror; but he will see his image in a diminished form, hanging upside down, and suspended in the air, in a line between his eye and the mirror. In this case, his image is formed _before_ the mirror as represented at IM fig. 22. In this situation, if you hold out your hand towards the mirror, the hand of the image will come out towards your hand, and, when at the centre of concavity, it will be of an equal size with it, and you may shake hands with this aerial image. If you move your hand farther, you will find the hand of the image pass by your hand, and come between it and your body. If you move your hand towards either side, the hand of the image will move towards the other side; the image moving always in a contrary direction to the object. All this while the by-standers, if any, see nothing of the image, because none of the reflected rays that form it can enter their eyes.--The following figure represents a phenomenon produced in the same manner. A B is a concave mirror of a large size; C represents a hand presented before the mirror, at a point farther distant than its focus. In this case, an inverted image of the hand is formed which is seen hanging in the air at M. The rays C and D go diverging from the two opposite points of the object, and by the action of the mirror, they are again made to converge to points at O and S where they cross, form an image, and again proceed divergent to the eye.[10] [Illustration: _figure 25._]