1] APPLICATION AND MEASUREMENT OF LIGHT 159 



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

 of relative warmth ; also the curves of relative warmth, brightness, and actinism 

 of the prismatic 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 BUXSEN 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 ordiuates is 

 entirely arbitrary. 



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

 at about X = 0.42yu, 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 



