on the Northern Hemisphere at Summer Solstice, 853 



P denoting the atmospherical pressure, and the constant 

 318*3 corresponding to the photochemical intensity of the 

 sunlight at the uppermost limit of of the atmosphere, while 

 the constant (•'4758 denotes the atmospherical extinction of 

 the direct solar light (PoggendorfY's Annalen, Bd. cviii. 

 p. 257 : Ostwald's Klassiker Ausgaben, Xo. 38, pp. 90, 91). 

 Developing in a series with increasing powers of cos <£, and 

 calculating the coefficients by means of the method of least 

 squares. Ban sen and Boscoe transformed the said formula to 



W : 31'99cos 2 <£ + 417'6cos 3 <j> — 24:&7 cos 4 <f>. . (b) 



To get the chemically active * quantity of light falling upon 

 a horizontal element of the surface of earth for a whole day, 

 we may substitute 



co> (f> = ees 8 cos p cos t + sin 8 sin p, 



p denoting the latitude, 8 the declination of the sun, and t 

 the hour angle of the sun. Then we may calculate the 

 integral 



W = | l wdt, 



where t y and — t Y denote the hour angle of the sun at 

 sunset and at sunrise respectively on the day in question. 

 F<»r the sake of shortness put 



sinSsinp=a and cosScos^>=/3, 



when the quantity under the integral may be written 



M7 = 31'99(a 2 + 2a#COS* + /3 2 COS 2 \ 



+ 4:17-6 (* 3 +3« 2 y3 cos t -f occ/3' 2 cos 2 1 + /3 3 cos 3 1) \ 

 - 24<s- 7 (a 4 + 4a 3 /3 cos t + 6V£ 2 COS 2 t + 4a£ 3 COS 3 t ' ' 



+/8*co8*it) 



At the equinoxes, when a=0, /3=cosp, and the integration 



is to be performed betweeu the limits £== — - and t= -f - , 



the said integral will attain a very simple form. Bunsen 

 and Roscoe calculated it to 



W= -1152<)cos 2 y>+127i;onco> :i / >-G7140cos 1 / y. 



Bunsen and Boscoe further determined the photochemical 



effect of the light reflected from a cloudless sky. They 



- By chemical action we understand hereafter only the action od the 

 explosive mixture of chlorine and hydrogen. 



Phil. Mag. S. 6. Vol. 9. No. 51. March 1905. 2 A 



