Page:Popular Science Monthly Volume 88.djvu/447

 Popidar Science Monthly

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��Why Is the Sky Blue?

Sunlight, which we call white, is com- posed of light rays of different colors — red, orange, yellow, green, blue, indigo and violet, ft can be broken up into its constituent colors in various ways. If it passes through a transparent prism (like the crys- tals that hang from a chande- lier) or if it falls on a surface which has almost invisibly mi- nute irregularities (like mother- of-pearl or the wing of a butter- fly ) we see the rays into which sunlight has been separated. These phenomena are observed when light is not absorbed.

Hold a piece of red glass in front of a flame and we see only red. Rays of all other colors have been absorbed. The natu- ral colors of the objects we see about us, leaves, flowers, books and chairs, depend upon absorp- tion. A green leaf throws back chiefly green rays ; the rest are absorbed. So, the natural color of everything in nature is the unabsorbed residue from full white light. There is no such thing as color by itself.

A swarm of minute particles, scattered in the path of white light, will break it up, like the surface of mother-of-pearl. If the particles happen to be of just the right size and the spaces between them just the right distance, they will absorb rays of one color only and throw off the rest. The atmosphere is filled with countless dust particles, and their size and spacing is such that they scatter rays which we call sky blue. Nearer the horizon, larger particles turn the blue into white ; this happens above a dusty town and when mists or clouds hang above us. All that is left of white sun- light, after passing throtigh many miles of blue-scattering air. appears in the hues of sunset. The size and spacing of dust particles as well as the angle at which sunlight strikes them, determines the color of the sky.

On the moon where there is no atmos- phere and no dust, the sky is jet black at noon. The sun appears as a vividly glowing disk in an inky canopy. That is also true of the vast space which exists between the stars.

��A Stairway Which Is Also a Door

IX order to construct a stairway be- tween floors in a limited space, a swinging stairway has been developed which does awav with the usual double-

���A stairway which has a hinged door section,

by which the cellar or the upper floor can

be reached with equal facility

width landing. The stairs are built with a hinge half way between the upper floor and the lancling, the landing being half way between floors. The stairs from the landing to the floor are built directly beneath the others. A person descending, stops at the landing to dis- engage a small catch. The released catch allows the lower portion of the hinged .stairs to fold upwards, so that the person passes underneath them to the lower staircase. A heavy weight makes it easy to lift the stairs when the catch is released.

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