;">f)2 B. T. LOWNE ON INTERFERENCE PHENOMENA. 



it came from / ; that which falls on B C, L B b, is reflected as 

 if it came from /'. So the screen between d and e is illuminated 

 by light, as if from two sources, / and V. As every wave is split 

 into two parts, whatever its phase, we may consider I and V as 

 two exactly similar sources of light, one the exact image of the 

 other ; each with exactly the same luminous particles in similar 

 positions with regard to each other, one emitting light always in 

 the same phase as that emitted by the corresponding particle in 

 the other, at every instant. 



It is obvious that no two natural sources of light could do this, 

 and that no two particles emitting light could have such a relation 

 to each other. 



The portion of the screen P Q between d and e which is illumi- 

 nated by rays which may be considered, as I have shown, to be 

 proceeding from the points I and I 1 (which are in every respect 

 identical), will exhibit alternate bright and dark bands, parallel 

 with the line of junction of the metallic mirrors A B, B C. The 

 screen will exhibit a black band wherever the distance from I 

 to the screen is -J, J, f of the length of a light wave longer or 

 shorter than the distance from h, and a bright band, wherever the 

 two paths are equal or differ by the length of 1, 2, 3, 4, etc., light 

 waves. In the former case the crest of a wave from I falls into the 

 hollow of the wave from /', and their joint effect is that no dis- 

 turbance or wave is propagated from the point, in the latter a 

 crest corresponds with a crest and the action of the two waves is 

 added, so that the light proceeding from the point is the same as 

 that produced by the two waves. 



II. On Diffraction. 



There is a well-known difference between light and sound ; if we 

 admit a beam of light into a dark room through a hole in a shutter, 

 the beam only illuminates the wall exactly opposite the opening, 

 whilst in the case of sound the disturbance is equally propagated 

 in all directions after passing through the aperture. The reason 

 is that the opening is very large in proportion to the magnitude of 

 light vibrations and very small in proportion to sound vibrations. 

 If the size of the aperture be reduced in the case of light so as to 

 be small in proportion to the size of light waves, then light, like 

 sound, is diffused after passing through the opening. Such 

 diffused rays of light are said to be diffracted at the opening. 



Not only is light diffracted in passing through an opening, but 



