She seems completely isolated in mid-air. She turns about, sometimes in a circle, moving now the legs, then the arms. Then after several graceful evolutions in all directions, she stands straight and descends rapidly, seemingly precipitated into a decorated scenery representing the ocean. [Illustration] The illusion is produced in this way: Behind a well-stretched muslin curtain, M M, is painted D D, with the sky and clouds, below a canvas representing the sea. In front, in the direction of G G, is a mirror, without quicksilver back, inclined at an angle of forty-five degrees. Below the mirror is a round table moving on a pivot, and on this the actress, who takes the part of the Amphitrite, lays down. In executing various movements, the table in turning, reflects in the glass the image of the person on whom a vivid light is thrown. The spectators placed at S see the image on the canvas at the back, D D. When the time comes for making the lady disappear altogether, the table, which glides on rails, is drawn off the stage, and Amphitrite seems to plunge into the waters. It is by this process that the specters and ghosts at the theaters are produced. You can perform this illusion, based on the reflection of the light at home, in reducing its construction to the simple proportions of a small theater of marionettes. Optical Illusions. Illusions of the eye are numberless, and afford a wide field for experiment. For example, if you ask any one wearing a silk high hat, to what height he thinks his hat would reach if placed on the ground against the wall or door. Nine times out of ten the mark of the height guessed will be half as much again, at least a third over the real height of the hat. [Illustration] Again, represents two triangles. Ask which is the one whose center is the better indicated. Every one will say, “triangle A.” Well, every one will be wrong, it is B. Take a pair of compasses and you will easily prove it. [Illustration] The same occurs with the above figure. The two parallelograms, A B, are absolutely equal, and yet A appears to be larger than B. The two lines, A and B are both of equal length; yet B seems a third longer than A. The sides, AB, CD, BD of the middle figure, BE, AM, EM, etc., are equal, yet it seems to the eye that the surface, A B E M, is longer than the square A B C D. There is another deception the eye is liable to. On a sheet of paper trace several circles, having the same center. Place the sheet on your thumb and turn it horizontally, it will then seem to you as if the rounds turned, though you watch with the utmost attention, the illusion will be complete. [Illustration] In order to terminate this series, which can be varied infinitely, we will, in our turn, ask you this question: Which is the tallest man of the three personages appearing in the adjoining figure? Is it the first, the last, or the middle one?