Popidar Science Monthly 



419 



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. 



