Fig. 3.
The annexed figure will better explain the direction in which to look: here, e, is the eye of the spectator, p, is the prism, h, the hole in the shutter or pasteboard, s, the spectrum. By a little practice, you will soon become accustomed to look in the right direction, and will see the colours very bright and distinct.
By means of this simple contrivance, white light may be analysed and proved to consist of coloured rays, and several of its properties be beautifully illustrated.
OPTICAL AUGMENTATION.
Take a glass rummer that is narrow at bottom and wide at top, into which put a half-sovereign, and fill the glass three-fourths with water; place on it a piece of paper, and then a plate, and turn the glass upside down quickly, that the water may not escape: by looking sideways at the glass, you will perceive a sovereign at the bottom, and higher up the half-sovereign, floating near the surface. Fill the glass with water, and the large piece only will be visible.
GOLD FISH IN A GLASS GLOBE.
A single gold fish in a globe vase, is often mistaken for two fishes, because it is seen as well by the light bent through the upper surface of the water, as by straight rays passing through the side of the vase.
COLOURS PRODUCED BY THE UNEQUAL ACTION OF LIGHT UPON THE EYES.
If we hold a slip of white paper vertically, about a foot from the eye, and direct both eyes to an object at some distance beyond it, so as to see the slip of paper double, then, when a candle is brought near the right eye, so as to act strongly upon it, while the left eye is protected from its light, the left-hand slip of paper will be of a tolerably bright green colour, while the right-hand slip of paper, seen by the left eye, will be of a red colour. If the one image overlaps the other, the colour of the overlapping parts will be white, arising from a mixture of the complementary red and green. When equal candles are held equally near to each eye, each of the images of the slip of paper is white. If, when the paper is seen red and green by holding the candle to the right eye, we quickly take it to the left eye, we shall find that the left image of the slip of paper gradually changes from green to red, and the right one from red to green, both of them having the same tint during the time that the change is going on.