PRESrCIPLE OF INTERFERENCES. 



ic- 



The interferences of liquid 

 waves are finely illusti'ated in 

 tile undulations of mercury con- 

 tained in a vessel of elliptical 

 fij^ure. If a disturbance be pro- 

 duced at one of the focal points of 

 the ellipse, the circular waves pro- 

 ceeding from it will, by reflection 

 from the sides of the vessel, form 

 a second similar system having 

 for its centre the other focus. If 

 the corresponding- j)oints of in- 

 ferference be connected, they will 

 form, as the figure shows, two 

 sets of curves, elliptical and hy- 

 perb lie, having for their com- 

 mon foci the foci of the original 

 ellipse. 



The interference of waves of 

 sound is often very perceptible. It is observed only in musical sounds because 

 it can only be observed in those whose undulations are continuous and uniform; 

 and such sounds are musical. It is best observed when the waves are long — as 

 in the case of the grave tones of the heavier organ-pipes. The sniking and 

 swelling of the sound, called by musicians the beat, is owing to one of the inter- 

 fering waves being slightly longer or shorter than the other. In many r-p<'ti- 

 tions this slight difference of length accumulates until it reach'^s half an undu- 

 lation, when, if the two waves originally conspired — that is, (to borrow agdn an 

 illustration from the water,) if their two crests were originally superposed — th y 

 will, after this difference has crept in, be in conflict ; or the crest of one will fall 

 lapon the hollow of the other. Daring this interval a sinking of the sound will 

 have been ob-^erved ; but immediately after, as the difference of path goes on 

 increa-iing from a half to a wliole undulation, the sound will swell asriin as the 

 two crests once more approach superposition. We need hardly remark that the 

 interference of waves of sound of prrfrrtli/ equal length would not be jicrcepti- 

 ble to us ; for, in that case, the resultant sound would be a constant. If we en- 

 deavor, by moving about while two bodies of precisely similar pitch are sound- 

 ing, to pass from the points of conspiring to those of conflicting undulation, we 

 shall not find it easy to detect these points for several reason.s 



In the first place, Avhen the molecular movements are normal to the wav 

 front, as in the case of sound, there is no complete interfiTence, or approach to 

 complete interference, except when the waves are tangt-ntial, or approximately 

 so, to each other ; except, therefore, in or near the line of the centres, and except, 

 it may be added, when the distance between the centres is an exact number of 

 half undulations. Again, at the intersections of sonorous waves, whether the 

 molecular movements conspire or conflict, their resultant is never so great as the 

 sum, nor so small as the di'iference of the two components. The difference of 

 intensity between the maxima and minima of sound in such cases will not be 

 striking, unless they succeed each other with brief intervening intervals of time, 

 as in the case of the heats* 



It is, however, by this second method that we detect the interferences of light, 

 and not at all by the first. That is to say, we discover these interferences by 

 moving the eye through the space where they exist, when the points of maxi- 

 mum and minimum brightness are easily observed ; or we let fall the interfering 



* Mr. Dcspr.-tz has succeeded in this rather difficult experiment of localizing the inter- 

 ferences of sound from two pipes in perfect uaisoa. 



