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 b c, will appear at a b.