158 J. U. IIAEFOED POLAEISATION OF LIGHT. 



timed intervals. This may be demonstrated by reference to the 

 colours of jS^ewton's rings, the successive intervals of thickness 

 producing a repetition of the same order of colours being in direct 

 arithmetical progression — 1, 2, 3, 4, etc; also by the effect of the 

 passage of light through a narrow slit. This is easily demon- 

 strated practically, but can hardly be well explained without a 

 diagram. In looking through a narrow slit the figure of the 

 opening will be seen covered with narrow dark lines, due to the 

 interference of waves of light passing through, or what is called 

 diffracted, from the boundary edges of the slit. 2nd. The theory 

 of the vibrating movement of particles of luminiferous ether trans- 

 verse to the line of propagation has already been refeiTed to. It 

 is probable and natural that a longitudinal alternate compression 

 and dilation should take place in the ether analogous to the action 

 of air in sound. Each half- wave is contrary in phase to the adjoin- 

 ing half, consequently under such circumstances the interference of 

 waves of light results in darkness. This may be illustrated by 

 means of a diffraction slit, or Prauenhofer's gratings, and also by 

 the dark intervals between the successive Ne-wton's rings, which 

 are explained by Fig. 12. 3rd. It is as difficult to conceive of the 

 possibility of the emission of paiticles from a luminous body at 

 the speed of 190,000 miles per second as it is of the emission of 

 particles of matter from a sounding body to the ear. The wave 

 theory seems simpler and more satisfactory, to say nothing of other 

 reasons. The tvave theory of light addresses itself to our know- 

 ledge, the emission theory to our ignorance. 4th. The interference 

 of Avaves of light at the half- wave interval results in the extinction 

 of light ; but at intermediate points the extinction is partial in 

 proportion to the amount of interference, and fringes of colour 

 result. These seem due to the effect on the eye of the residue of 

 the vibrations and their continuous impression on the sensation of 

 sight. This is somewhat analogous to the beats of sound which 

 are heard Avhen two musical notes of discordant pitch are sounded 

 together. The coincidences produce an augmented sound, and 

 when folloAving slowly may be counted, but if in rapid succession 

 produce a sustained effect on the ear. 



On the principle of interference may be explained most of the 

 phenomena of colour in ordinary light as well as those observed in 

 polarised light. In all cases one ray of light is found or made to 

 run concurrently with another in such a manner that the phases of 

 the vibrations shall in some part counteract or neutralise one 

 another, and the resulting vibrations produce the effect of colour 

 as being part only of the combined effect of the total vibratit)ns 

 constituting the wave. For instance, in the case of thin films part 

 of the light is reflected at the first surface, another part enters the 

 film and is reflected from the second interior surface, and emerges 

 in close proximity witli the first reflected beam (Fig. 13). If the 

 dittbrence owing to the interiuil ri^fraction and reflexion amounts to 

 a com])lete and exact wave, the two waves of retlt'xion will ((incur 

 in tlieir respective phases, — no inteiference will take place, and the 



