VISIBLE AND NEAR-VISIBLE RADIATION 



171 



recently, aluminum mirrors have been developed commercially Avhich 

 exhibit a much higher reflectance over the entire range (better than 80 per 

 cent in the ultra-violet, and exceeding slightly tarnished silver through- 

 out the visible). Quantitative data, however, are not yet available. 



If a diffusing surface is desired, powders may be applied to a tacky 

 varnish surface (Fig. 16, shows the reflectance of the various materials). 



'2,000 2,400 2,800 3.200 3.600 4,000 4,400 4800 5,200 5,600 6,000 



Wave Length 



Fig. 17. — Reflectance of commercial paints. (Luckiesch, 47.) 



In case commercial paints are used, selection may be made in accordance 

 with Fig. 17 (47). 



SOURCES OF CONTINUOUS EMISSION 



SUN 



Despite the extensive development of artificial sources of radiation, 

 even for experimental purposes the sun must be considered a source of 

 primary importance, because of its high intensity and desirable wave- 

 length distribution. The chief difficulties which one encounters in 

 using this source arise from the great variability of the radiation as it 

 reaches the surface of the earth. The causes of this variability are as 

 follows: variation in distance from sun to earth; influence of the earth's 

 atmosphere, affected by altitude or air mass; variability of water vapor 

 and ozone ; and the presence of materials due to artificial conditions such 

 as dust and smoke. 



During a clear summer day at sea level (Washington, D. C), the total 

 solar radiation received on a horizontal surface varies according to the 

 dotted curve in Fig. 18, a record from the October, 1924, Monthly 

 Weather Review by Kimball (43, cf. page 478). 



A portion of this total radiation is due to scattering from the sky. 

 While the sky radiation attains a maximum at noon, its relative con- 



