Interference Phenomena, 



541 



during which a more or less regular motion is produced in 

 our eye by the action of light. We may easily distinguish 



Fig. 



7. 



between yellow and orange light, the periodic times of which 

 stand in the ratio of 61 to 59. The fraction (T a — TJ/Ti is 

 about g 1 ^, and the number of vibrations during which the 

 effects of an impulse will last must at least contain 24 com- 

 plete periods. On the other hand, 1 think we should find it 

 difficult to distinguish in any part of the spectrum between 

 two vibrations the periods of which differ by the three- 

 hundredth part of either ; and we may conclude that the 

 physical effects of an impulse in the eye could not show any 

 regularity lasting for much more than 300 periods. We are 

 not of course dealing here with the physiological actions 

 which prolong the sensation of light for a measurable time. 



The fact that white light shows any objective interference 

 at all, without the artificial introduction of regularity, is due 

 to the prevalence of certain wave-lengths over others. What- 

 ever regularity there is in the light is intimately connected 

 with the distribution of intensity in the spectrum. 



We cannot help speculating as to the ultimate cause which 

 renders the regularity of vibration a function of the tempe- 

 rature only, and independent of the natural periods of the 

 molecules. Perhaps the solution of the difficulty will be 

 found in the fact that our observations tell us nothing directly 

 as to the vibrations of the atoms or molecules. What we 

 observe is the disturbance of the medium, and the distribu- 

 ton of energy in the spectrum of an incandescent black body 

 which is in thermal equilibrium may indicate a property of 

 the medium rather than that of matter. That is to say, 

 the motion of vibration in the molecule may be perfectly 



Phil. Mag. S. 5. Yol. 37. No. 229. June 1894. 2 P 



