Part 19 (1/2)

[Ill.u.s.tration: Fig. 25.]

_To show the Spots in the Sun's Disk by its Image in the Camera Obscura._

Put the object-gla.s.s 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 Gla.s.s._

By the diagonal position of a plane mirror, a curious opera-gla.s.s is constructed, by which any person may be viewed in a theatre or public company without knowing it. It consists only in placing a concave gla.s.s near the plane mirror, in the end of a short round tube, and a convex gla.s.s in a hole in the side of the tube, then holding the end of the tube with the gla.s.s to the eye, all objects next to the hole in the side will be reflected so as to appear in a direct line forward, or in a position at right angles to the person's situation who is looked at. Plane gla.s.ses, instead of a convex and concave, may be used; in this case the size of the object will not be increased, but it will appear brighter.

_To observe an Eclipse of the Sun, without Injury to the Eye._

Take a burning-gla.s.s, or spectacle-gla.s.s, that magnifies very much; hold it before a book or pasteboard, twice the distance of its focus, and you will see the round body of the sun, and the manner in which the moon pa.s.ses between the gla.s.s and the sun, during the whole eclipse.

_The Burnt Writing restored._

Cover the outside of a small memorandum book with black paper, and in one of its inside covers make a flap, to open secretly, and observe there must be nothing over the flap but the black paper that covers the book.

Mix soot with black or brown soap, with which rub the side of the black paper next the flap; then wipe it clean, that a white paper pressed against it will not receive any mark.

Provide a black-lead pencil that will not mark without pressing hard on the paper. Have likewise a small box, about the size of a memorandum book, and that opens on both sides, but on one of them by a private method. Give a person a pencil and a slip of thin paper, on which he is to write what he thinks proper; you present him the memorandum book at the same time, that he may not write on the bare paper. You tell him to keep what he writes to himself, and direct him to burn it on the iron plate laid on a chafing-dish of coals, and give you the ashes. You then go into another room to fetch your magic box, before described, and take with you the memorandum book.

Having previously placed a paper under the flap in the cover of the book, when he presses hard with the pencil, to write on his paper, every stroke, by means of the stuff rubbed on the black paper, will appear on that under the flap. You therefore take it out, and put it into one side of the box.

You then return to the other room, and taking a slip of black paper, you put it into the other side of the box, strewing the ashes of the burnt paper over it. Then shaking the box for a few moments, and at the same time turning it dexterously over, you open the other side, and show the person the paper you first put in, the writing on which he will readily acknowledge to be his.

If there be a press or cupboard that communicates with the next room, you need only put the book in the press, and your a.s.sistant will open it, and put the paper in the box, which you presently after take out, and perform the rest of the amus.e.m.e.nt as before.

There may likewise be a flap on the other cover of the book; and you may rub the paper against that with red lead. In this case you give the person the choice of writing either with a black or red pencil; and present him the proper side of the book accordingly.

_The Opaque Box made Transparent._

Make a box three or four inches long, and two or three wide, and have a sort of perspective-gla.s.s, the bottom of which is the same size with the box, and slides out, that you may privately place a paper on it.

The sides of this perspective are to be of gla.s.s, covered on the inside with fine paper.

Let a person write on a slip of paper, putting your memorandum book under it, as in the last amus.e.m.e.nt; then give him the little box, and let him put what he has written into it. In the mean time you put the memorandum book into the press, where the perspective is already placed. Your a.s.sistant then takes the paper out of the book, and puts it at the bottom of the perspective; which you presently take out of the press, and direct the person to put the little box that contains the paper under it. You then look in at the top of the perspective, and feigning to see through the top of the box, you read what is written on the paper at the bottom of the perspective.

With this perspective box you may perform another amus.e.m.e.nt, which is, by having in a bag twelve or more ivory counters, numbered, which you show to the company, that they may see all the numbers are different.

You tell a person to draw any one of them, and keep it close in his hand. You then put the bag in the press, when your a.s.sistant examines the counters, and sees which is wanting, and puts another of the same number at the bottom of the perspective, which you then take out, and placing the person's hand close to it, look in at the top, and pretending to see through his hand, you name the number on the counter in it.

_The Transposable Pieces._