1 58 APPARENT SIZE OF SUN AND MOON. SECT. XVIII. 



atmosphere, increases with the obliquity of incidence. Of ten 

 thousand rays falling on its surface, 8123 arrive at a given point 

 of the earth if they fall perpendicularly ; 7024 arrive if the angle 

 of direction be fifty degrees ; 2831, if it be seven degrees ; and 

 only five rays will arrive through a horizontal stratum. Since 

 so great a quantity of light is lost in passing through the atmo- 

 sphere, many celestial objects are altogether invisible from 

 the plain, which may be seen from elevated situations. Dimi- 

 nished splendour, and the false estimate we make of distance from 

 the number of intervening objects, lead us to suppose the sun 

 and moon to be much larger when in the horizon than at any 

 other altitude, though their apparent diameters are then some- 

 what less. Instead of the sudden transitions of light and dark- 

 ness, the reflective power of the air adorns nature with the rosy 

 and golden hues of the Aurora and twilight. Even when the 

 sun is eighteen degrees below the horizon, a sufficient portion of 

 light remains to show that at the height of thirty miles "it is 

 still dense enough to reflect light. The atmosphere scatters the 

 sun's rays, and gives all the beautiful tints and cheerfulness of 

 day. It transmits the blue light in greatest abundance ; the 

 higher we ascend, the sky assumes a deeper hue ; but, in the 

 expanse of space, the sun and stars must appear like brilliant 

 specks in profound blackness. 



