Astronomical and Nautical Collections, 129 



nevertheless be perfectly reconciled, as we have seen, with 

 the regular continuance of a great number of oscillations in 

 each of the series separated by these perturbations. This 

 being admitted, it is impossible to suppose that these per- 

 turbations should take place simultaneously and in the same 

 manner in the vibrations of separate and independent par- 

 ticles ; so that it will happen, for example, that the motions 

 of the, one will be retarded by an entire semioscillation, while 

 those of the other will be continued without interruption, 

 or will be retarded by a complete oscillation, a change 

 which will completely invert the whole effects of the inter-* 

 ference of the two systems of undulations Avhich originate 

 from them; since ifthey had agreed on the first supposition, 

 they would totally disagree on the second. Now these oppo- 

 site effects, succeeding each other with extreme rapidity, will 

 produce in the eye a continuous sensation only, which will 

 be a mean between the more or less lively sensations that 

 they excite, and will remain constant, whatever may be the 

 difference of the routes described. 



But the case is different when the two luminous pencils 

 originate from a common source : for then the two systems 

 of waves, having originated from the same centre of vibra- 

 tion, undergoing these perturbations in the same manner and 

 at the same instant, undergo no changes in their relative po- 

 sitions : so that if they disagreed in the first instance at any 

 given point, they would continue to disagree at all other 

 times ; and if their motions cooperated at first, they would 

 continue to agree as long as the centre of vibration continued 

 to be luminous : so that in this case, the effects must remain 

 constant, and must therefore be sensible to the eye. This 

 is therefore a general principle, applicable to all the effects 

 produced by luminous undulations ; that in order to become 

 sensible, they must be permanent. 



We have hitherto supposed that the two systems of waves 

 were moving exactly in the same direction, and tliat conse- 

 quently their elementary motions, to be combined with 

 each other, were precisely limited to one single line : this is 

 the simplest case of interference, and the only one in whicH 

 the one motion can be completely destroyed by the other: 



JULY— OCT. 1827. K 



