Opaque Bodies Seemingly Transparent





Within the case A B C D, place four mirrors O P Q R, Fig. 22, so

disposed, that they may each make an angle of 45 degrees, that is,

that they may be half-way inclined from the perpendicular, as in the

figure. In each of the two extremities A B, make a circular overture;

in one of which fix the tube G L, in the other the tube M F, and

observe, that in each of these is to be inserted another tube, as H

and I. [Observe. These four tubes must terminate in the substance of

the case, and not enter the inside, that they may not hinder the

effect of the mirrors. The four-fold reflection of the rays of light

from the mirrors, darkens in some degree the brightness of the object;

some light is also lost by the magnifying power of the perspective.

If, therefore, instead of the object-glass at G, and concave eye-glass

at F, plain glasses were substituted, the magnifying power of the

perspective will be taken away, and the object appear brighter.]






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.





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