322 POPULAR LECTURES^ND ADDRESSES. 



undulations extending from red to violet in the 

 spectrum (which I am going to show you pre- 

 sently), I would first point out on this chart 

 (Fig. 48) that in the section from letter A 

 to letter D we have visual effect and heating 

 effect only ; but no ordinary chemical or photo- 

 graphic effect. Photographers can leave their 

 usual sensitive chemically prepared plates exposed 

 to yellow light and red light without experiencing 



FIG. 48. The Solar Spectrum. 



any sensible effect ; but when you get toward the 

 blue end of the spectrum the photographic effect 

 begins to tell, and more \\ arid \ more strongly as 

 you get towards the violet fend. When you get 

 beyond the violet there is th^ invisible light known 

 chiefly by its chemical action.' From yellow to 

 violet ,we have visual effect^' I heating effect, and 

 chemical effect, all three ; above .the violet only 

 chemical and heating effects, and so little of the 

 heating effect that it is scarcely perceptible. 



