§ 1] APPLICATION AND MEASUKEMENT OP LIGHT 159 



Fig. 42. — Scale of normal solar spectrum, above which is drawn the normal curve 

 of relative warm.th ; also the curves of relative warmth, brightness, and actinism 

 of the pHsmatic solar spectrum. The curves of warmth are taken from Langley 

 ('84, p. 233) ; the curve of brightness is constructed from the data of Vierordt 

 ('73, p. 17) ; that of actinism is taken from Bunsen and Roscoe ('59, p. 268) and 

 indicates the relative efficiency of the different rays of the midday sun in caus- 

 ing the union of chlorine and hydrogen. The absolute value of the ordinates is 

 entirely arbitrary. 



a longer wave length than 0.51 yu, have feeble chemical action ; 

 at about X=:0.42^ this action reaches a maximum. 



Not only the quality but also the intensity of the light with 

 which we experiment must be known. It is, fortunately, quite 



