Imágenes de páginas
PDF
EPUB

upon a black cloth laid upon a table in a dark room, and on top of the mouth is laid a postal-card, having a slit, 1 inch (25 millimetres) long and (1 millimetre) wide, cut in it. Above the jar is a

inch

[graphic][merged small]

hand-mirror, so placed that the beam of sunlight from the heliostat (or from a hole in the curtain) will be reflected downward upon the postal-card on top of the jar.

This simple apparatus is designed to show how

light is reflected from small particles floating in the air. Set fire to a small bit of paper and drop it into the jar. Place your hand over the mouth of the jar, and in a moment it will be filled with smoke. When the paper has burned out, put the postal-card in place, so that the slit will be in the centre of the mouth of the jar. Let the beam of reflected light from the mirror fall on this slit.

Look in the jar and you will see a slender ribbon of light extending downward through the jar. Elsewhere it is quite dark and black. Here we see the light streaming through the opening in the card, and lighting up the particles of smoke in its path.

Take off the card, and let the reflected beam fall freely into the jar. The smoke is now wholly illuminated, and the jar appears to be full of light, and every part of the bottle shines with a pale-white glow.

Put the postal-card on again and let the light fall through the slit. The smoke has nearly all disappeared, and the ribbon of light in the jar is quite dim. Curious streaks and patches of inky blackness run through it. What is this? Nothing-simply nothing. The smoke is melting away, and the beam of light disappears because there is nothing to reflect it

and make it visible.

This part of the experiment appears quite magical in its effects, and is exceedingly interesting.

THE MILK-AND-WATER LAMP.

Take away the jar and put a clear glass tumbler in its place. Fill this with water and throw the beam of reflected light down upon it, and the water will be lighted up so that we can easily see the tumbler in the dark. Now add a teaspoonful of milk to the water and stir them together. Throw the beam of light down once more. This is indeed remarkable. The tumbler of milk-and-water shines like a lamp, and lights up the room so that we can easily see to read by its strange white light. Move the mirror and turn aside the beam of light, and instantly the room becomes dark. Turn the light back again, and once more the glass is full of light.

Here the minute particles of milk floating in the water catch and reflect the light in every direction, so that the entire goblet seems filled with it, and the room is lighted up by the strange reflections that shine through the glass.

CHAPTER IV.

REFRACTION OF LIGHT.

CERTAIN things, like glass, water, mica, and ice, allow light to pass directly through their substance. We hold them before the eyes, and see the light very nearly as well as through the air. Such substances, we say, are transparent. Other objects, like porcelain or oiled paper, do not permit all the light to pass, and such things, we say, are semitransparent or translucent. Many other things do not permit light to pass through them, and cast shadows behind them when brought into a beam of light. These things cut off all the light, and we call them opaque.

Here is a common glass bottle with straight sides and about three inches (76 millimetres) broad, or as wide as a postal-card (Fig. 14). On one side is pasted a piece of white paper having a perfectly round hole cut in it. On the glass, in the clear space made by the circular opening in the paper, are two lines drawn at right angles, in ink. These two lines divide the circle into four equal parts, and are to serve as guides in some new experiments.

Fill the bottle with clear water up to the hori

[graphic][merged small]

zontal line in the circle, and then, holding the bottle in a small horizontal beam of sunlight from the heliostat, you will see that the light passes directly through the water in the bottle or through the air above the water. To make this more distinct, cut a

« AnteriorContinuar »