INTERFERENCE OF LIGHT. 83 



lights must emanate from a common origin, the vibrations 

 of two independent sources of light not being synchronous. 

 The simplest mode of effecting this is to condense the Sun's 

 rays into a small luminous point by means of a convex lens 

 of short focus.* 



(103) We shall best understand the circumstances of this 

 phenomenon, by considering what takes place in another 

 more familiar case of interference. If two stones be flung at 

 the same instant into a pool of stagnant water, a series of 

 circular waves will be propagated from each of the two cen- 

 tres of disturbance ; and where these two sets of waves cross, 

 they will produce effects similar to those we have been de- 

 scribing in the case of light. Where the crest of one wave 

 falls upon the crest of another, they will be added together, 

 and form a higher crest, or ridge, on the surface. And, on 

 the contrary, where the crest of one wave meets the hollow, 

 or sinus, of another, they will counteract one another's effects, 

 and the water will stand at that point at its original level, as 

 if undisturbed. 



It is obvious that there will 

 be several sets of consecutive 

 points of each class, or several 

 lines of double disturbance and 

 no disturbance. One line of 

 double disturbance, AA, will be 

 produced by the meeting of waves 

 equidistant from the two centres ; 



* A convenient mode of performing the experiment of Young has been 

 employed by M. Billet. A convex lens, of tolerably long focal length, is cut into 

 two halves, which are reunited in such a manner that their optical centres are 

 slightly separated. Under these circumstances the rays diverging from a distant 

 point, and falling on the lens, will converge to two near points, and will be in a 

 condition to interfere after divergence. The intensity of the light is much 

 greater than that emanating from two small apertures. 



G 2 



