< Page:Popular Science Monthly Volume 11.djvu
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THE POPULAR SCIENCE MONTHLY.

vessels appear repeated in the sky, or when distant land that is really below the horizon 'looms' up and glimmers upon the horizon in trembling headlands. This illusion is called the mirage, and takes place when refraction exceeds the critical angle and becomes reflection. "Fill a clear glass tumbler with water, and put a spoon in it, or dip one finger in the water, and hold it above your head, so that you can look into the water from below. You will find that you cannot see through the water up into the air above. The under-surface of the water will appear to shine like burnished silver, and the spoon or your finger will be reflected in it, as in a beautiful mirror. This illustrates total reflection, and shows that in this case all light thrown upward through the water is reflected from its surface. Look into the tumbler from above, and it appears full of clear water. Look into it from below, and it seems as if an opaque sheet of silver rested on the water, and shut out the view of everything above."

PSM V11 D684 Water lens.jpg

Fig. 7.—The Water-Lens.

Fig. 7 shows a neat and simple arrangement by which water can be used for a lens to illustrate refraction. It is merely a long pine box fourteen inches high, made of thin boards, with one side open, and a round hole in the top five inches in diameter. On this opening rests a hemispherical glass dish made by cutting off the round top of a glass shade, and which is filled with clear water. A piece of looking-glass is supported below at an angle of forty-five degrees, so as to reflect a stream of sunlight upward through the water-lens. The rays are thus refracted and brought to a point at a proper distance above. If a sheet of paper is held horizontally just over the bowl, it will be lighted up by the rays coming through the lens. Raise the paper slowly, and the circle of light on its surface will grow smaller and brighter, till it is reduced to a point, when it will burn a hole in the paper. If a little smoke is diffused through the space, it will reveal the double cone of light, with one base upon the surface of the water-lens, and the other forming a large circle of light upon the ceiling, the rays all crossing at the focal point. With some additional attachments Prof. Mayer makes such a lens work in a magic lantern for projecting large pictures upon screens, the whole mechanism being estimated to cost but $3.20.

One of the most beautiful experiments in total reflection is that illustrated by Fig. 8. A Florence flask filled with water acts as a

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