THE LEAF AXC ITS FUNCTIONS 



71 



is due to niinut(> uiul (MioihiousIn- rapid vilji'aiioiis, (he Icii^lh of 

 the vibration — its wave-length — dctcnnininp; the color of the 

 Hght produced. Sunhght, or any white hght, is composed of 

 vil)rations of a great variety of (Uflerent wave-lengths, but when 

 such light is passed through a prism these become sorted out into 

 a many-colored spectrum. The visible spectrum runs from the 

 red rays, the wave-length of which is approximately 750 millionths 

 of a millimeter, to the violet ones, where it is approximately 

 400. These visible radiations are by no means the only ones 

 which occur, however. Rays of longer wave-length than red — 



P'iG. 37. — Diagrams of a spectroscope, showing light broken up by a prism into 

 its constituent portions, which form a spectrum. A, spectrum of white light. 

 B, spectrum of light which has passed through a chlorophyll solution, showing the 

 dark absorption bands in the red and the blue. {From Ganong, "The Living 

 Plant", Henry Holt and Co.) 



the infra-red rays — pass gradually into heat-waves, and those 

 shorter than violet — the ultra-violet rays — are active chemically. 

 When falling upon different objects, light behaves differently. 

 All of it may be absorbed by the object and converted into heat 

 or some other form of energy, the object then appearing black; 

 or all may be reflected, the object then appearing white; or 

 certain wave-lengths may be absorbed and certain others reflected, 

 the object in such a case displaying to our eyes the color of 



