SECT. XX. INTERFERENCE OF LIGHT. 167 



SECTION XX. 



Interference of Light Undulatory Theory of Light Propagation of 

 Light Newton's Rings Measurement of the Length of the Waves of 

 Light, and of the Frequency of the Vibrations of Ether for each Colour 

 Newton's Scale of Colours Diffraction of Light Sir John Her- 

 schel's Theory of the Absorption of Light Refraction and Reflection of 

 Light. 



NEWTON and most of his immediate successors imagined light 

 to be a material substance, emitted by all self-luminous bodies 

 in extremely minute particles, moving in straight lines with pro- 

 digious velocity, which, by impinging upon the optic nerves, 

 produce the sensation of light. Many of the observed pheno- 

 mena have been explained by this theory ; it is, however, totally 

 inadequate to account for the following circumstances. 



When two equal rays of red light, proceeding from two lumi- 

 nous points, fall upon a sheet of white paper in a dark room, 

 they produce a red spot on it which will be twice as bright as 

 either ray would produce singly, provided the difference in the 

 lengths of the two beams, from the luminous points to the red 

 spot on the paper, be exactly the 0'0000258th part of an inch. 

 The same effect will take place if the difference in the lengths 

 be twice, three times, four times, &c., that quantity. But 

 if the difference in the lengths of the two rays be equal 

 to one-half of the 00000258th part of an inch, or to its 

 ! 2, 3^, &c., part, the one light will entirely extinguish 

 the other, and will produce absolute darkness on the paper 

 where the united beams fall. If the difference in the 

 lengths of their paths be equal to the li, 2, 3, &c., of the 

 0'0000258th part of an inch, the red spot arising from the com- 

 bined beams will be of the same intensity which one alone would 

 produce. If violet light be employed, the difference in the 

 lengths of the two beams must be equal to the 0'0000157th 

 part of an inch, in order to produce the same phenomena ; and 

 for the other colours, the difference must be intermediate be- 

 tween the 0'0000258th and the 0'000015 7th part of an inch. 



