24i Oil the Relative Intensities of Sunlight and Daylight j <^t. 



altitude of 25° 16', the mean ratio of the chemical intensities of direct and 

 diffused light being 0-23, that of the luminous intensities was 4-00, or that 

 the action of the atmosphere was 1 7*4 times greater on the chemically 

 active than on the luminous rays of sunlight. A series of photometrical 

 experiments made afterwards at Owens College gave the following re- 

 sults : — 



Mean altitude of the sun 1 2° 3' 



Mean ratio of chemical intensity .... 0'053 

 Mean ratio of luminous intensity, . . . 1*400 

 It appears therefore that with the sun at an altitude of 12° 3', the action 



of the atmosphere was 26*4 greater on the chemical than on the luminous 



rays. 



The foregoing experiments appear to prove — 



I. That the effect of the atmosphere upon the highly refrangible and 

 chemically active solar rays is regulated by totally different laws from 

 those founded upon the hypothesis of the reflexion by means of hollow 

 vesicles of water. 



II. That the ratio of the chemical intensity of direct to diffuse sunlight 

 for a given altitude of the sun at different localities is not constant, vary- 

 ing with the transparency, &c., of the atmosphere. 



III. That this ratio of ''chemical" intensity does not in the least cor- 

 respond to the ratio of "visible" intensity as estimated by the eye; the 

 action of the atmosphere being 17'4 times greater upon the chemical than 

 on the luminous rays when the sun's altitude is about 25° 16', and 26*4 

 times greater when the sun's altitude is 12° 3'. 



OCT, 8. .oGG. 



