Atmospheric Optical Illusions 99 
FURTHER CONSIDERATIONS 
The fact that the sky appears blue in color, and that at 
sunset the sun is a deep red and illuminates clouds with a 
gorgeous red and yellow glow, is due to scattering of light by 
tiny particles in the atmosphere—microscopic dust particles 
and presumably even the air molecules themselves. Because 
the particles are small they are able to scatter effectively only 
the shorter wave lengths of light—the violet and the blue. 
The shorter wave lengths of light are scattered in all direc- 
tions so that the sky has a bluish color no matter in which 
way the observer looks. This leaves only yellow and red light 
coming directly from the sun, so that the sun appears golden. 
At sunrise or sunset the light of the sun passes through more 
air than when overhead, and then only the red light comes di- 
rectly from the sun, all else being scattered, so that near the 
horizon the sun appears a deep red. . 
In conclusion we might mention some effects which are 
purely psychological in nature, or due to perspective. If two 
parallel bands of clouds pass across the sky, they will appear 
to meet at either horizon, although being parallel they actu- 
ally do not meet. The same illusion is observed when distant 
ships send up parallel searchlight beams, extending from 
horizon to horizon. The beams arch across the sky, are 
furthest separated directly overhead and appear to meet at 
both ends. 
The sun and moon appear to be much larger at the horizon 
than when overhead; however, when measured by a sextant, 
or photographed, the difference in size is seen to be purely 
psychological. The apparent flattening of the disks is real 
_ enough, being due to differential atmospheric refraction. 
