Furnish the first of these tubes with an object-glass at G, and a concave eye-glass at F. You are to observe, that in regulating the focus of these glasses with regard to the length of the tube, you are to suppose it equal to the line G, or visual pointed ray, which entering at the aperture G is reflected by the four mirrors, and goes out at the other aperture F, where the eye-glass is placed. Put any glass you please into the two ends of the moveable tubes H and L; and lastly, place the machine on stand E, moveable at the point S, that it may be elevated or lowered at pleasure.
When the eye is placed at F, and you look through the tube, the rays of light that proceed from the object T, passing through the glass G, are successively reflected by the mirrors O P Q and R to the eye at F, and there point the object T in its proper situation, and these rays appear to proceed directly from that object.
The two moveable tubes H and I, at the extremity of which a glass is placed, serve only to disguise the illusion, for they have no communication with the interior of the machine. This instrument being moveable on the stand E, may be directed to any object; and if furnished with proper glasses, will answer the purpose of common perspective.
The two moveable tubes, H and I, being brought together, the machine is directed towards any object; and, desiring a person to look at the end F, you ask him if he sees that object distinctly. You then separate the two moveable tubes, and, leaving space between them sufficiently wide to place your hand or any other solid body, you tell him that the machine has the power of making objects visible through the most opaque body; and as a proof, you desire him to look at the same object, when to his great surprise he will see it as distinctly as if no solid body interposed.
This experiment is the more extraordinary as it is very difficult to conceive how the effect is produced; the two arms of the case appearing to be made for the purpose of supporting the perspective-glass; and to whatever object it be directed, the effect is still the same.
The Deforming Mirrors.
If a person look in a concave mirror placed perpendicularly to another, (that is, supposing one mirror to be laid on the floor, and the other attached to the ceiling,) his face will appear entirely deformed. If the mirror be a little inclined, so as to make an angle of 80 degrees, (that is, one-ninth part from the perpendicular,) he will then see all the parts of his face, except the nose and forehead. If it be inclined to 60 degrees; (that is, one-third part,) he will appear with three noses and six eyes: in short, the apparent deformity will vary at each degree of inclination, and when the glass comes to 45 degrees, (that is, half-way down,) the face will vanish. If, instead of placing the two mirrors in this situation, they are so disposed that their junction may be vertical, then different inclinations will produce other effects, as the situation of the object relative is quite different.
The Magic Tube.
Procure a small tube of glass, whose canal is extremely narrow, and open at both ends; let one end of it be plunged in water, and the water within the tube will rise to a considerable height above the external surface: or if two or more tubes be immersed in the same fluid, the one with a narrow canal, and the other wider, the water will ascend higher in the former than the latter.