THE SKY 55 



often afford opportunities for interesting observations. It will 

 often be found then that during the middle of the day the sun 

 penetrates easily through the mist-laden atmosphere, and we 

 have a characteristic pale autumn sky. As it sinks down 

 towards evening its rays can no longer easily penetrate the 

 atmosphere, and the evening mist seems to gather early. 



If will hardly be possible to explain the optical reason why 

 the sky during the day varies from blue to grey according to the 

 amount of solid matter in suspension, but the fact that it does 

 contain such particles should at least be demonstrated. The 

 familiar experiment of allowing a beam of sunlight to penetrate 

 through a hole in the shutter into a dark room should be per- 

 formed, and the dancing motes pointed out. Explain that air 

 always contains such dust particles, and that they are less in the 

 country than in the town, less on high snow mountains than near 

 dusty roads, but are always present in greater or less amount. 

 Try to suggest also that this dust is most plentiful in the lower 

 layers of the atmosphere, in the layers which the slanting rays of 

 the setting sun must penetrate. It is for this reason that when 

 lofty mountains are ascended the sky becomes a deeper and a 

 purer blue, ultimately darkening almost to indigo. Speak also 

 of the effect of the minute drops of water or even of the minute 

 crystals of ice which may be suspended in the upper air. 



As to the exact phenomena of sunset, the chief points to 

 be noted are the following. As the sun sinks the variety of 

 colour increases, but its intensity diminishes. Two sets of 

 colours appear ; the one, the glow colours, are arranged round 

 the sun, while the second or horizon colours spread north and 

 south of the setting sun. The glow colours centre round a point a 

 little above the sun, which becomes red or yellow as it sinks instead 

 of the dazzling white of noonday. The glow colours pass from 

 silvery white through yellow to rosy pink, the last reaching 

 to about 25 degrees above the sun. Let the pupils hold a 

 protractor level with the setting sun, and by aid of a ruler 

 endeavour to calculate the angular extension of the colours. The 

 horizon colours, when at their fullest development, may extend 

 to some 60 or 80 degrees from the sun. An attempt should 

 be made to measure this point also. The horizon colours 



