AMUSEMENTS OF SCIENCE.

BY

PETER PARLEY, THE YOUNGER.

PART V.

I. OPTICAL AMUSEMENTS.
II. GEOMETRICAL AMUSEMENTS.
III. MATHEMATICAL AMUSEMENTS.
IV. ON MEASUREMENT OF TIME.

E have already shown that optical amusements are very pretty and instructive, as light and vision are of the greatest importance to us, and furnish topics of investigation highly interesting. The science of optics has of late been made use of to furnish ghosts to the

British public, which has set all London in a ferment. The principle upon which ghosts are made may be easily gathered.

EXPERIMENT 1.

Place two plane mirrors about eight inches high and six in width, in a box as in the annexed Fig. ; the edges being neatly joined and the mirrors standing at an angle of ninety degrees,—that is, at a right angle, with respect to each other. The experiment succeeds better if the top of the box is covered in. The effect of this arrangement is singular: if a person looks in that side of the box which is open, the two mirrors, if neatly joined, will appear as one, and the spectator will be surprised to find that if he raises his right hand to his head, his reflected image will appear to raise the left hand in the same manner. This is caused by the image which is received by the right hand mirror being reflected in the first instance to that on the left, which by a second reflection conveys it to the eye of the spectator.

That beautiful instrument, the kaleidescope, is formed by a peculiar arrangement of two oblong plane mirrors, in a metal

or pasteboard tube. In forming one of these curious and amusing instruments, it is necessary that the two mirrors should be so placed that the distance between the edges A and B should be an even or an odd part of the circumference of the tube in which they are placed, and the plates of glass

must be about six times as long as they

are wide. In using the instrument, it is necessary that the eye should be placed exactly in the centre of the circle at one

end of the tube; and the object that is to form the picture, close to the mirrors at the other end.

The effect produced by the reflecting powers of concave mirrors is, under certain circumstances, exceedingly curious, and at first sight inexplicable. If a number of parallel rays of light reach a concave mirror, A B, they will be reflected from that mirror, and meet

in a point at F: this point is called the focus of the mirror, and is always at the distance of one-half the radius of the circle, of which the mirror forms a part, from the face of the mirror. If a glass bottle, half full

A

B

F

of water, is held before a concave mirror at a greater distance from it than its focus, and the spectator retires a

short distance, the image of the bottle will appear reversed and seem to be in front of the mirror.

But the most singular thing is that the water will appear

in the image, not to occupy its usual place, but to fill that end of the bottle nearest the neck, while the part it really does occupy will appear empty. If the bottle is reversed (of course well corked) the water will naturally run to that part which is lowest, namely the neck; but in the reflected image it will appear to occupy the bottom of the bottle, which is in reality empty, and becoming more so, will, on the contrary, seem to be filling; but as soon as all the liquid has run out the illusion ceases, and the bottle appears to be empty. The effect produced by this experiment is simply an illusion of the mind, arising from the knowledge we possess of the properties of liquids to remain at the lowest part of any vessel which may contain them, assisted also by the colourless nature of water; for if a coloured liquid is employed, this illusion does not take place.

A very beautiful illustration of the properties of the concave mirror, is shown at most of our optical exhibitions, which, when well done, produces a most perfect illusion.

A concave mirror, A B, is placed behind a black screen, in

the screen a small bracket is placed, supporting a flower pot filled with earth or moss. If an observer stands at some distance from the screen, with his eye on a level with the hole, a beautiful image of the flower will appear as if

springing from the flower-pot, and so distinct that you might almost suppose you could touch it.

GEOMETRICAL AMUSEMENTS.

All triangles whose base and perpendicular are equal, are equal in their superficial contents, whatever may be their

angles. Thus the triangles, A B C, C D E, and E F G are equal to each other.

To form two squares of unequal size into one square, equal to both the original squares.-Suppose the upper part of Fig. 2 (on the next page) to represent two squares of unequal size: lay them close together as in the engraving, mark off the length of one side of the smaller square on the top of the larger; from A to B draw one line from B to C, and another from B to D; cut the pasteboard from B to D, and from B to C; remove the triangle, A B D; place it with its side A D against the bottom of the larger square, so that the line D A shall touch it, the angle A being to the left. If now the triangle B E C is also removed, and placed with its side E C against the bottom line of the smaller square, it will be found that the five pieces of pasteboard will form a perfect square, equal in its superficial contents to the two original squares.