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begins and another ends. Yet with a little care you can make out a number of colors, that seem quite distinct. Seven colors can very easily be counted by beginning at the red, or left end of the spectrum. These colors are red, orange, yellow, green, blue, indigo, and violet. Some people count ten, and call them red, orange, yellow, yellowish-green, green, bluish - green, ultramarine - blue, indigo, violet, and lavender. All these colors, and the countless shades of colors that lie between them, are in the beam of sunlight. Decompose the light by means of a prism, and they stand side by side on the screen in a beautiful band or belt. Each color has a different degree of refraction, the refraction increasing from the red to the violet; and thus they meet the screen at different places, and we see them spread out side by side like a band or ribbon
the screen. To prove that this solar spectrum is the solar light decomposed, and to show that all these colors may be found in a beam of white light, place a handmirror in the beam of refracted light just beyond the prism. The spectrum may thus be reflected to a distant part of the room, on the wall or ceiling. Then, holding the mirror in the fingers, make it vibrate to and fro, so that the reflected spectrum will move, in the direction of its length, from side to side very
quickly. At once the spectrum on the wall changes into a streak of white light, with colored spots at each end. To understand this, you must remember the common experiment of whirling a lighted stick or bit of live coal. The spot of fire changes into a ring of light. When light falls upon
its effect lingers for a short time, even after the source of the light has moved away, or has ceased to give light. The vision is said to persist or stay after the light has really gone. So in this case the colors of the moving spectrum on the wall persist or stay in the eye while they are moving to and fro, and thus one color overlaps another, and we seem to see them all at once in one place. This mingling of every color in the eye gives us a band or streak that appears white, and thus, indirectly, proves that all the colors of the spectrum make white, and that white light contains all these colored lights. · At the ends of this band of white are bright spots of color. As the mirror moves backward and forward, it stops at each end of its little journey to change its direction, and here the spectrum becomes visible.
EXPERIMENTS WITH THE SOLAR SPECTRUM.
Send to the dealer in artists' materials and get a cake of red vermilion, emerald-green, and aniline violet (Hoffman's violet-B. B.). If this color cannot be found, buy “Nuremberg violet.” Get these shades, and no others, and then cut out three narrow strips of cardboard, and give one a coat of the red, one a coat of violet, and paint the other green. Take pains to give them a good thick coat, so as to hide the white paper. Study the solar spectrum on the screen carefully, and you will see that these shades of red, green, and violet, are in it. When the painted strips are dry, take the red vermilion strip and hold it in the spectrum at the left or red end, and you will see that it matches the red exactly. Tip the paper backward a trifle so that the surface of the paper will not shine or glisten in the light, and then move it slowly to the right, keeping it before the spectrum. As it passes the orange it grows dark; in the yellow it is darker still ; opposite the green it is perfectly black. Move it to the very end, and everywhere the red strip is quite black. Place it before the red again, and its color comes out clear and bright. Try the violet strip in the same way. In the same manner, the green strip is green when it is in the green part of the spectrum, and black everywhere else.
This experiment shows that green, red, and violet, are visible in green, red, and violet light, and that in light of any other color they are invisible, and the strip of card appears to be black. Hence, an object appears of its proper color because it absorbs all colors of the white light except its own color which it reflects.
Look at the spectrum closely, and you will notice that the red is at one end, the green near the middle, and violet is at the other end. Between the red and the green you will notice many shades of yellow, from deep-orange to yellowish-green, and between the green and the violet are many shades of blue, from greenish-blue to deep indigo.
It is thought that, when we see a red light, certain nerves in the eye are affected, and convey a peculiar sensation to the brain, that we call red. These nerves are sensitive to red light, but are not sensitive to any other light, except in a moderate degree. Another set of nerves in the eye are peculiarly sensitive to green light, and still another set are affected by violet light. Hence the sensations caused by these three colors are called the three elementary color sensations, and from the combinations of these sensations come all countless shades of color. When one of these colors falls on the eye, we see it distinctly. When two say the red and green meet the eye, both sets of nerves are affected at
once, and we get a sensation that is neither red nor green, but yellow. In the same manner, when green and violet meet in the eye, the two sets of nerves are excited, and we see not green and violet, but blue. In the same manner, if red, green, and violet light enters the
all the nerves are excited at once, and we see not three colors, but one, which is white.
This diagram will assist us to remember the relation the color sensations bear to each other. The red and the green combine to make yellow; the green and the violet unite to make blue; all three mingled