LECTURE VIL 

 ON LIGHT. 



PART II. THEORIES OF LIGHT INTERFERENCES 



DIFFRACTION.. 



r O theories only, entitled to any consideration 

 as rational and intelligible explanations of 

 the phaenomena of Light, have been ad- 

 vanced the one proposed by Sir Isaac 

 Newton, commonly known as the "Corpuscular;" the 

 other by Christian Huyghens, as the " Undulatory" 

 theory. According to the former, light consists in 

 " Corpuscules," or excessively minute material particles 

 darted out in all directions from the luminous body, 

 in virtue of some violent repulsive power, or other en- 

 ergetic form of internal action, acting under such cir- 

 cumstances, and under such laws, as to give them all the 

 same initial velocity which they retain unchanged in their 

 progress through space, as well as their initial direction 

 according to the general laws of motion (to all which 

 they implicitly conform), until they meet with some mate- 

 rial body by whose action their course is changed. All 



