72 



THE MECHANISM OF LIFE 



On the contrary, the hypothesis that light is propagated by 

 vibratory movements is open to many objections. Even the 

 Zeeman effect, although it may tend to establish the fact that 

 light is produced by vibratory movement, by no means proves 

 that it is propagated in the same manner. When the theory- 

 was accepted that the transmission of light was periodic it 

 was supposed that this periodic transmission could only be 

 vibratory or undulatory in character, since waves or vibrations 

 were the only periodic phenomena known at that time. We 

 now know that there are other means of periodic transmission 

 which are apparently not undulatory. The periodic precipitates 

 produced by diffusion show us the transmission of spherical 

 waves through space, which follow the laws of light, although 



the periodic phenomenon is 

 apparently emissive rather 

 than vibratory. 



It will be remembered 

 that Newton considered light 

 to he produced by projectile- 

 like particles emanating from 

 a centre, and proceeding in 

 straight lines in all directions. 

 This emission theory of light 

 was abandoned in favour of Huygens' undulatory theory. 



It was said that the phenomena of interference and diffrac- 

 tion could not be explained by the theory of emission, while 

 the undulatory theory gave a simple explanation. The 

 scientific mind was unable to conceive the idea of emission 

 and periodicity as taking part in the same phenomenon. 

 The savants and thinkers who have meditated on this question 

 have always considered the theory of emission and that of 

 periodicity as incompatible. Nevertheless, we are here in 

 presence of a phenomenon in which emission and periodicity 

 exist simultaneously. The molecules emanating from our 

 drop are diffused in straight radiating lines, and vet produce 

 periodic precipitates which are subject to interference and 

 diffraction like the undulations of Iluvgens. 



The phenomena associated with the pressure of light, the 



Fig. 16. — Interference 

 waves. 



I .litfii 



