VISIBLE AND NEAR-VISIBLE RADIATION 



177 



Using the darkening of ZnS as a detector, Clark (18, cf. page 246) has 

 observed the ultra-violet, cut off by a microscope slide (see curve 15, 

 Fig. 9) for both daily and seasonal variation at Baltimore, Md. 



Figure 23 (18) shows the daily fluctuation for clear days at different 

 representative times of the year. It will be noticed on comparison with 

 Fig. 18 that the ultra-violet rises more sharply to a maximum and then 

 decreases more rapidly than the total radiation, owing to the greater 

 influence of atmospheric absorption. 



Oct Nov. Dec. Jnn. Feb. March April May June Julj( Aug, Sep. Oct. Nov Dec. Jan. Feb. March Apnl 



Fig. 24. — Seasonal variatioa in ultra-violet radiation. {Zinc sulfide measurements by 



Clark, 18.) 



The annual variation in ultra-violet radiation lags somewhat behind 

 that of the total radiation, as will be seen by comparing Clark's values, 

 Fig. 24, with those of Kimball (page 224). 



The variations in ultra-violet intensity and short-wave-length limit 

 have been attributed by many observers to the absorption of atmos- 

 pheric ozone. According to Dobson (18), the effective depth of ozone in 

 the atmosphere amounts to 0.3 to 0.5 cm. at normal temperature and 

 pressure. Figure 25 shows the seasonal fluctuation in effective depth 

 observed by Dobson at Oxford averaged over the years 1925 to 1928, 

 inclusive. The absorption coefficients for ozone under standard condi- 

 tions are given by curve C, upper section of Fig. 2. Corresponding 

 percentage transmissions are given for 1.0 cm. depth at the right. By 

 shifting this scale up one-half division, one obtains the value for 0.3 cm. 

 depth. This indicates an effective cut-off in the region of 2900 A, rising 



