LECTURE VII, 

 ON LIGHT. 



PART II. THEORIES OF LIGHT INTERFERENCES 

 DIFFRACTION. 



[WO 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 th.-ir 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 



