96 



INTERFERENCE OF LIGHT. 



To see this, let a bed represent a portion of a spherical wave ; 

 and let be the place 

 of the eye, and Oa the 

 line drawn from it to 

 the luminous centre. 

 Commencing from the 

 point a, let portions 

 ab, be, cd, &c., be 

 taken, such that the 

 differences of the dis- 

 tances of their extremities from the point shall be the same 

 for all, and equal to half a wave. Now we may suppose all 

 these portions of the grand wave to be so many centres of 

 disturbance ; and it is obvious that the secondary waves, sent 

 by each pair of consecutive portions to the eye, are in com- 

 plete discordance, and should wholly destroy one another if 

 their intensities were equal. It is easy to see that this is the 

 case with respect to portions, as /</, gh, which are remote from 

 the point a. For the magnitudes of the waves sent by the 

 several portions to any point depend first, on the magni- 

 tudes of these portions themselves, and secondly, on the 

 angles which the line drawn from them to that point makes 

 with the front of the wave. As respects the former, it is evi- 

 dent that ab is greater thn be, be than cd, and so on ; but 

 these differences continually diminish, and the magnitudes of 

 the consecutive portions approach indefinitely to equality, as 

 they recede from the point a. The same is true of the obli- 

 quities. Hence, the portions of the wave, fg, gh, remote from 

 the point , destroy one another's effects, and the effect 

 on the eye, or on a screen at 0, will be entirely due to those 

 parts of the grand wave which are in the neighbourhood of 

 the line connecting that point with the luminous origin. 



Of these parts ab produces the greatest effect, being both 

 the largest and the least oblique. The effect of the neigh- 



