Photometric Observations of the Sun and Sky. 



267 



which falls from the same surface of zenith sky, multiplied by the 

 preceding ratio, must give the chemical action which the whole sky 

 would produce on a horizontal unit of surface." 



I have below in one or two points only attempted to institute a 

 numerical comparison between the results of Sir H. Boscoe and my 

 own ; considering the great difference in our methods, I am not 

 surprised that no good coincidence in the results can be established. 



DIAGRAM 1. 



24. Having given i a the chemical action in the circle of minimum 

 intensity, to calculate the total chemical action of the sky on a plane 

 sxposed at right angles to the Sun. 



(N.B. ia is a constant for this calculation, but it varies with the 

 Ititude of the sun). 



Let the figure (Dia. 1) represent a projection on the plane of sym- 

 letry, S being the Sun, Z the zenith, HRYH' the horizon, AYX the 

 lane of minimum intensity, SH = the Sun's altitude. 

 Let 9 be the angular distance from the Sun of the elementary zone 



