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The Barometer: Experiment

A curious Experiment, of nearly the same kind as the last, called Optical Augmentation.

Take a large drinking-glass, of a conical figure, and having put a stilling into it, fill the glass about half full with water. then place a plate on the top of it, and turn it quickly over, so that the water may not get out. This being done, look through the glass, and you will now perceive a piece of money of the size of half-a-crown; and somewhat higher up, another piece of the size of a shilling. But if the glass be entirely filled with water, the large piece at the bottom only will be visible.

This phenomenon is occasioned by your seeing the piece through the conical surface of the water, at the side of the glass, and through the flat surface at the top of the water, at the same time; for the conical surface dilates the rays, and makes the piece appear larger, while the flat surface only refracts them, and occasions the piece to be seen higher up in the glass, but still of its natural sire.

Another curious Experiment, called Optical Subtraction. Against the wainscot of a room fix three small pieces of paper, as A, B, C, fig. 7, (see Plate,) about a foot and a half or two feet asunder, at the height of your eye; and placing yourself directly before them, about five times the distance from them that the papers are from each other, shut one of your eyes, and look at them with the other, and you will then see only two of those papers, suppose A and B; but altering the position of your eye, you will now see the third, and one of the first, suppose A; and by altering its position a second time, you will see B and C, but in neither case all three of them together.

The cause of this phenomenon is, that one of the three pencils of rays, which come from these objects, falls on the optic nerve at D, whereas, to produce distinct vision, it is necessary that the rays of light fall on some part of the retina E, F, G, H.

From this experiment, the use of having two eyes may be easily perceived; for he that has only one can never see three objects placed in this position; or all the parts of one object, of the same extent, without altering the situation of his eye. An Optical Experiment, shewing how to produce an Artificial Rainbow.

In any room which has a window facing the sun, suspend a glass globe, filled with water, by a string which runs over a pulley, so that the sun's rays may fall directly upon it; then drawing the globe gradually up, when it comes to the height of about forty degrees above the horizon, you will see, by placing yourself in a proper situation, the glass tinged with a purple colour; and by drawing it gradually higher up, the other prismatic colours, blue, green, yellow, and red, will successively appear; but after this they will all vanish, till the globe is raised to about fifty degrees, when they will again be seen, but in an inverted order, the red appearing first, and the blue, or violet, last; and when the globe comes up to little more than fifty-four degrees, they will entirely vanish. These appearances serve to illustrate the phenomena of natural rainbows, of which there are generally two, the one being about eight degrees above the other, and the order of their colours inverted, as in this experiment; the red being the uppermost colour in the lower bow, and the violet in the other.

An artificial Rainbow may also be produced as follows. Take some water in your mouth, and turn your back to the sun; then if it be blown forcibly out against some dark or shady place, you will see the drops formed by the beams of the sun into an apparent rainbow, which, however, soon vanishes.

A curious Optical Illusion, produced by means of a Concave Mirror.

Take a glass bottle, (see Plate,) ABC, fig. 8, and fill it with water to the point B; leave the upper part, BC, empty, and cork it in the common manner; place this bottle opposite a concave mirror, and beyond its focus, so that it may appear reversed; then if you place yourself still farther from the mirror, the bottle will appear to you in the situation a b c. And in this apparent bottle it is remarkable, that the water, which, according to the laws of catoptrics, and all other experiments of this kind, should appear at a b, appears, on the contrary, at b c, the part a b seeming to be entirely empty. And if the bottle be inverted, and placed before the mirror, as in the under part of the figure, its image will appear in its natural erect position, but the water, which is in reality at bc, will appear at a b.

And if, while the bottle is inverted, it be unco:ked, and the water suffered to run gently out, it will appear, that while the part BC is emptying, the part a b in the image is filling; and if, when the bottle is partly empty, some drops of water fall from the bottom A, towards BC, it seems in the image as if there were formed at the bottom of the part a b bubbles of air arising from a to b, which is the part that seems full. The circumstances most remarkable in this experiment, are, first, not only to see an object where it is not, but also where its image is not; and, secondi,, that of two objects, which are really in the same place, as the surface of the bottle and the water it contains, the one should be seen at one place, and the other at another; and also that the bottle should be seen in the place of its image, and the water where reither it nor its images are.

It is, however, to be noted, that if any coloured liquor he put into the bottle instead of water, no such illusion will take place.

There is one phenomenon more of this kind, which ought not to be omitted; for though it be common enough, it is also extremely pleasing, and easy to be performed.

If you place yourself before a concave mirror, at a proper distance, your figure will appear inverted; and if you stretch out your hand towards the mirror, you will perceive another hand, which seems to meet and join it, though imperceptible to the touch.

And if, instead of your hand, you make use of a drawn sword, and present it in such a manner that its point may be directed towards the focus of the rays reflected by the mirror, another sword will appear, and seem to encounter that in your hand. But it is to be observed, that to make this experiment succeed well, you must have a mirror of at least a foot in diameter, that you may see yourself in part; and if you have a mirror large enough to see your whole person, the illusion will be still more striking.

How to make a violent Tempest, by means of artificial Rain and Hail.

Make a hollow cylinder of wood, very thin at the sides, about eight or ten inches long, and two or three feet in diameter. Divide its inside into five equal partitions, by means of boards of about six inches wide; and let there be a space between them and the wooden circle, of about one-sixth ot an inch; observing, that the boards are to be placed obliquely to each other.

This being done, put into the cylinder four or five pounds of leaden shot, of a size that will easily pass through the opening left for this purpose; then turn the cylinder on its axis, and the sound of the machine, when in motion, wil. represent that of rain, which will increase with the velocity of the motion; and if a larger sort of shot be used, it will produce the sound of hail.