Construct a machine similar to that in Fig. 25. Its effect will be, that a print, or an ornamented drawing, with any object, such as an ace of diamonds, &c. in the centre F, will be seen as an ace of clubs when placed in the machine, and viewed through a single plane glass only, contained in the tube E. The glass in the tube F, which produces this surprising change, is somewhat on the principle of the common multiplying glass, as represented at G, which, by the number of its inclined surfaces, and from the refractive power of the rays proceeding from the objects placed before it shows it in a multiplied state. The only difference is, that the sides of this glass are flat, and diverge upwards from the base to a point in the axis of the glass like a cone; it has six sides, and each side, from its angular position to the eye, has the property of refracting from the border of the print F, such a portion of it (designedly placed there) as will make a part in the composition of the figure to be represented; for the hexagonal and conical figure of this glass prevents any part of the ace of diamonds being seen; consequently the ace of clubs being previously and mechanically drawn in the circle of refraction in six different parts of the border, at 1, 2, 3, 4, 5, 6, and artfully disguised in the ornamental border, by blending them with it, the glass in the tube at E will change the appearance of the ace of diamonds, F, into the ace of clubs, G. In the same manner many other prints undergo similar changes, according to the will of an ingenious draughtsman who may design them. The figure of the glass is shown at H. [Illustration: Fig. 25.] _To show the Spots in the Sun's Disk by its Image in the Camera Obscura._ Put the object-glass of a ten or twelve feet telescope into the scioptric ball, and turn it about till it be directly opposite the sun. Then place the pasteboard mentioned in page 16, in the focus of the lens, and you will see a clear bright image of the sun, about an inch diameter, in which the spots on the sun's surface will be exactly described. As this image is too bright to be seen with pleasure by the naked eye, you may view it through a lens whose focus is at six or eight inches distance, which, while it prevents the light from being offensive, will, by magnifying both the image and the spot, make them appear to greater advantage. _The Diagonal Opera Glass._