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Another kind of slide may be made by flowing skimmed milk over sheets of glass. When the white film of milk is dry, drawings may be traced in it with a sharp pencil or pointed stick. Another plan is to rub Castile soap over glass, and to draw on this in the same way. By this plan you can destroy the picture by rubbing on more soap, and you may then make a new picture in it.
This lantern is quite as good as the best magic lanterns. For schools, where one boy or girl wishes to show a sum in arithmetic, an example in algebra, a map, or sample of penmanship, to the whole school, the sun-lantern and piece of smoked glass, or a sheet of gelatine, will enable him to project it on a screen, so that a hundred boys and girls can see it at once.
Another interesting experiment may be made with this lantern by taking the glass cover off of the large lens, and dropping a very small chip of wood in the water. It will be pictured in gigantic size upon the screen, and curious fringes of shade will gather round it, showing where the water clings to the wood. A drop of camphor or of oil of coriander or oil of cinpamon, let fall into the water, will exhibit geometrical figures and strange motions on the screen; and a few drops of indigo or carmine ink will color the screen blue or red, and make an excellent background for some of the pictures.
To describe all that could be done with this waterlantern and heliostat would fill a book. Having made them, you can consult other books on making projections, and find the lantern a source of amusement and instruction for hundreds of people for a very long time.
THE SOLAR MICROSCOPE.
Fig. 25 represents a common round glass flask, about 6 inches (15.3 centimetres) in diameter; a common pocket-microscope lens of 1 inch focus (costing 25 cents), and a glass slide, carrying a microscopic object. The flask is filled with water, and is placed on a table just at the opening of the heliostat, so that the light will be refracted in it and brought to a focus. It is thus a water-lens, and may be used to bring a focus of light upon any object placed near it. Just behind this focus we place a glass slide, containing some object to be examined in a microscope. To hold this slide upright, we stick it in a mass of wax. The magnifying-glass is fastened to a bit of wax resting on a block of wood, so that it may be moved backward or forward along a strip nailed down on another block. About 15 feet (4.57 metres) from the 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.
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 sunlight may be split into a vast number of rays, each having a color of its own. This beautiful band of color is called the solar spectrum. Study it carefully.