table is placed a screen, and on this is projected a large image of the minute object on the slide. A cloth is hung over the upper part of the water-lens, to shut out the light, and all other light is excluded from the room. This apparatus makes a solar microscope, that may be used to project all kinds of micro

scopic objects, so that they can be exhibited before a large number of people. Tanks for holding animalcules, and all objects used in microscopes, may be placed in this solar lantern and exhibited upon a large scale, with very little trouble, and at only the expense of the flask, the pocket-lens, the screen, and the heliostat.

CHAPTER V.

THE DECOMPOSITION OF LIGHT.

CUT a vertical slit, an inch (25 millimetres) long and of an inch (1 millimetre) wide, in a piece of cardboard. Make the slit with sharp, clean edges, and then fasten the cardboard over the opening in the heliostat, and a slender ribbon of light will enter the dark room. In front of this slit place a small block of wood, and on this put a lump of wax. At the optician's you can purchase for 50 cents a good glass prism. Stand this upright in the wax, as in Fig. 26. Behind this, at a distance of about 15 feet (4.57 metres), hang up the screen we used in the lantern projections.

Here A is the opening in the heliostat, but somewhat exaggerated in size. The prism is at P, and S shows how the screen is placed, but gives its position much too near the prism.

On the screen will be projected a band of brilliantcolored light, resembling the rainbow. We have seen that light may be reflected, and that it may be refracted; here we discover that, by refraction, it may be decomposed-that a single beam of white sun

light may be split into a vast number of rays, each This beautiful band of

having a color of its own.

color is called the solar spectrum. Study it carefully.

VIBGYOR

FIG. 26.

It is quite impossible to count the colors, for they mingle together and merge into each other by invisible gradations, so that we cannot say where one color

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 upon 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 the eye 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 vio