436 HISTORY OF OPTICS. 



aware of the true ground of the difference of the 

 result in the cases of sound and light; namely, 

 that any ordinary aperture bears an immense ratio 

 to the length of an undulation of light, but does not 

 bear a very great ratio to the length of an undula- 

 tion of sound. The demonstrable consequence of 

 this difference is, that light darts through such an 

 orifice in straight rays, while sound is diffused in 

 all directions. Euler, not perceiving this difference, 

 rested his answer mainly upon a circumstance by 

 no means unimportant, that the partitions' usually 

 employed are not impermeable to sound, as opake 

 bodies are to light. He observes that the sound 

 does not all come through the aperture; for we 

 hear, though the aperture be stopped. These were 

 the main original points of attack and defence, and 

 they continued nearly the same for the whole of 

 the last century ; the same difficulties were over 

 and over again proposed, and the same solutions 

 given, much in the manner of the disputations of 

 the schoolmen of the middle ages. 



The struggle being thus apparently balanced, 

 the scale was naturally turned by the general as- 

 cendancy of the Newtonian doctrines; and the 

 emission theory was the one most generally adopted. 

 It was still more firmly established, in consequence 

 of the turn generally taken by the scientific activity 

 of the latter half of the eighteenth century; for 

 while nothing was added to our knowledge of op- 

 tical laws, the chemical effects of light were studied 



