SECT, xx.] INTERFERENCE OF LIGHT. 187 



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 Herschel'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-lu- 

 minous bodies in extremely minute particles, moving in 

 straight lines with prodigious velocity, which, by impinging 

 upon the optic nerves, produce the sensation of light. Many 

 of the observed phenomena 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 

 luminous 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 dif- 

 ference in the lengths of the two beams, from the luminous 

 points to the red spot on the paper, be exactly the 00000258th 

 part of an inch. The same effect will take place if the dif- 

 ference in the lengths be twice, three times, four times, <fcc. 

 that quantity. But, if the difference in the lengths of the two 

 rays be equal to one-half of the 0'0000258th part of an inch, 

 or to its l, 24r, 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 1 J, 2J, 3J, &c., of 

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

 the combined beams will be of the same intensity which one 



