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]
figure 25.
In consequence of the properties of concave mirrors, now described, many curious experiments and optical deceptions have been exhibited. The appearance of images in the air, suspended between the mirror and the object, have sometimes been displayed with such dexterity and an air of mystery, as to have struck with astonishment those who were ignorant of the cause. In this way birds, flying angels, spectres and other objects have been exhibited, and when the hand attempts to lay hold on them, it finds them to be nothing, and they seem to vanish into air. An apple or a beautiful flower is presented, and when a spectator attempts to touch it, it instantly vanishes, and a death’s head immediately appears, and seems to snap at his fingers. A person with a drawn sword appears before him, in an attitude as if about to run him through, or one terrific phantom starts up after another, or sometimes the resemblances of deceased persons are made to appear, as if, by the art of conjuration, they had been forced to return from the world of spirits. In all such exhibitions, a very large concave mirror is requisite, a brilliant light must be thrown upon the objects, and every arrangement is made, by means of partitions, &c., to prevent either the light, the mirror, or the object from being seen by the spectators. The following representation (fig. 26.) shows one of the methods by which this is effected: A is a large concave mirror, either of metal or of glass, placed on the back part of a dark box, D is the performer, concealed from the spectators by the cross partition C; E is a strong light, which is likewise concealed by the partition I, which is thrown upon the actor D, or upon any thing he may hold in his hand. If he hold a book, as represented in the figure, the light reflected from it will pass between the partitions C and I to the mirror, and will be reflected from thence to Z, where the image of the book will appear so distinct and tangible, that a spectator looking through the opening at X, will imagine that it is in his power to take hold of it. In like manner, the person situated at D, may exhibit his own head or body—a portrait, a painting, a spectre, a landscape, or any object or device which he can strongly illuminate.
figure 26.
figure 27.
figure 28.
