56 ACTINISM OF SOLAR LIGHT. PART i. 



produced on this standard paper by a constant quantity 

 of light, the tint being- the same, whether light of the 

 intensity represented by 1 acts for the time represented 

 by 50, or light represented by 50 acts for the time 

 represented by 1 ; or in other words the amount of the 

 chemical action of light is directly proportional to the 

 intensity of the light, and when the light is constant, 

 the amount of action is exactly proportional to the time 

 of exposure. 



, The ratio of the chemical action of the rays of light 

 falling directly from the sun to the chemical action of 

 the light diffused over the whole sky can be determined 

 by means of an instrument, in which the shadow of a 

 little ball is made to fall on a sensitive paper so as to 

 intercept the direct rays of the sun, and allow it to 

 be impressed by an action of the light diffused over 

 sky alone ; this compared with a similar paper, on 

 which both the direct and indirect light has fallen, 

 gives the ratio required. From this it appears, that 

 the relative amount of chemically active light which 

 comes directly from the sun, is very much less than 

 the amount of his direct visible light. For while 

 Professor Eoscoe was making experiments at Manchester 

 on the maximum effect of the chemical action of light, 

 he found when the sun had an altitude of 20, that 

 of 100 chemical rays which fell on a piece of standard 

 paper, only about 8 came from the direct light of the 

 sun ; while on the contrary, of 100 rays of visible light, 

 66 came directly from the sun, and only 40 from the 

 light diffused over the whole sky, so that the diffused 

 light is richer in chemical rays than the direct solar 

 beam, ' a startling result,' but borne out by observations 

 not only made at Manchester and in its vicinity, but 

 at Kew, Heidelberg, and at Para on the Amazon nearly 

 under the equator. 



On account of the increasing rarity of the atmo- 



