VISIBLE AND NEAR-VISIBLE RADIATION 



175 



Forsythe and Christison (28), Fig. 30. The shortest detectable wave- 

 length is near 2900 A, varying markedly with locality air mass, and 

 season. Thus, Dorno reports for Davos (46, cf. pages 36, 53) that the 

 limit varies from 2973 A at midday in July to 3093 A in December. 



4.000 6.000 8,000 10,000 12.000 14,000 16,000 18,000 20,000 



Fig. 21. — Spectral distribution of radiation from the sun, outside the atmosphere, and 

 at air masses 1, 2, 3, 4, and 5, on a clear dry day at Montezuma, precipitable water 0.05 cm. 

 {From data of Abbot, 4.) 



o 



On August 29, 1909, the value at noon was 2963 A, while from 5 a.m. to 

 6 A.M., it was 3202 A. Even with sun at zenith, the fall in intensity in 

 proceeding to shorter wave-lengths is extremely great. Laurens (46, 

 cf. page 38) gives the value at 2929 A as only 1/10,000 of that 

 for 3140 A. Consequently, if one undertakes to evaluate intensities 



