Astronomical and Nautical Collections, 125 



as are composed of undulations of the same length; for if 

 the waves were longer one than the other, however small 

 their difference might be, it would happen at last that their 

 relative position would not be the same throughout the ex- 

 tent of the groups ; and while the first destroyed each other 

 almost completely, the following ones would be less in oppo- 

 sition, and would ultimately agree completely with each 

 other: hence there would arise a succession of weak and 

 strong vibrations analogous to the beatings which are pro- 

 duced by the coincidence of two sounds differing but little from 

 each other in their tone ; but these alternations of weaker and 

 stronger light, succeeding each other with prodigious rapi- 

 dity, would produce in the eye a continuous sensation only. 

 -»iflt is very probable that the impulse of a single luminous 

 semiundulation, or even of an entire undulation, would be 

 too weak to agitate the particles of the optic nerve, as we 

 find that a single undulation of sound is incapable of causing 

 motion in a body susceptible of a sympathetic vibration. 

 It is the succession of the impulse, which, by the accumu- 

 lation of the single effects, at last causes the sonorous body 

 to oscillate in a sensible manner ; in the same manner as the 

 regular succession of the single efforts of a ringer is at last 

 capable of raising the heaviest church bell into full swings. 

 Applying this mechanical idea to vision, supported as it is 

 by so many analogies, we may easily conceive that it is 

 impossible for the two remaining semiundulations, which 

 have been mentioned, to produce any sensible effect on the 

 retina ; and that the result of such a combination of the 

 two systems must be the production of total darkness. 



If again we suppose the second system of undulations to 

 be again retarded half an undulation more, so as to make 

 the difference of the progress an entire undulation,, the coin- 

 cidence in the motions of the two groups will be again 

 restored, and the velocities of oscillation will conspire and 

 be augmented in the points of superposition; the intensity 

 of the light being then at its maximum. ; , ;-> 



Adding another semiundulation to the difference in the 

 progress of the two systems, so as to make it an interval and 



