SECT. XX. INTERFERENCE OF LIGHT 161 



agination has a powerful influence on our optical impres- 

 sions, and has been known to revive the images of highly 

 luminous objects months, and even years, afterward. 



SECTION XX. 



Interference of Light Undulatory Theory of Light Propagation of Light 

 ings M 

 equency 

 ton's Scale of Colors Diffraction of Light Sir John Herschel's Theory 



gt ropagaon of ight 



Newton'* Rings Measurement of the Length of the Waves of Light, 



Ether for each Color New- 



and of the Frequency of the Vibrations of 

 ton's Scale of Colors Diffraction of Light 

 of the Absorption of Light Refraction and Reflection of Light. 



NEWTON and most of his immediate successors imag- 

 ined light to be a material substance, emitted by all self- 

 luminous bodies in extremely minute particles, moving 

 in straight lines with prodigious velocity, which, by im- 

 pinging upon the optic nerves, produce the sensation of 

 light. Many of the observed phenomena have been ex- 

 plained by this theory ; it is, howev,er, 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 difference in the lengths of the two'beams, 

 from the luminous points to the red spot on the paper, 

 bo exactly the 0-0000258th part of an inch. The same 

 effect wiU 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 0-0000258th part of an inch, or to its 

 H, 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 1|, 2|, 3|, 

 &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 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 colors, the difference must be intermediate be^ 



