ATMOSPHERE AND LIGHT 129 



ing from objects at a distance is so refracted and reflected by 

 the layers of air of different density, through which it comes 

 to the eye of the observer, that objects appear to be where 

 they are not, like the image of a person seen in a mirror. 

 This phenomenon is called mirage or looming. It occurs 

 most frequently on deserts and over the sea near the coast. 

 Sometimes in high latitudes arches and streamers of 

 colored light are seen illuminating the northern sky. The 



LICK OBSERVATORY. 



As light is affected by the atmosphere, observatories must be placed 



where atmospheric conditions are the best. This famous observatory is 



on a mountain in the clear air of California. 



brilliancy and colors of the illumination vary. Sometimes 

 it is bright enough to be seen even in the daytime. This 

 display is called the aurora borealis or " northern lights " 

 and is believed to be an electrical phenomenon in thin air. 

 The heights of the streamers have been calculated to be 

 more than a hundred, perhaps several hundred miles, so 

 that it is probable that air in a rare condition extends to 

 this elevation. 



