B. T. L0WNE ON INTERFERENCE PHENOMENA. 



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it is diffracted by opaque bodies when sufficiently small, so that a 

 minute globule of mercury produces a diffraction image consisting 

 of concentric rings of light and darkness when seen with the 

 microscope by illumination from below. These rings result from 

 diffraction and interference conjointly. 



The cause of diffraction may be explained as follows. 



Fig. 10. 



Suppose the light, Fig. 10, to proceed from a luminous point, it 

 will have a great wave front, A B, made up of thousands of small 

 wavelets. As these are close together, the lateral disturbances 

 annul each other. But now suppose a narrow grating, C D, inter- 

 posed with spaces just wide enough each to allow a single wavelet 

 to pass through, the light will radiate from every point in each 

 clear space ; that is, the light will radiate equally in all directions 

 beyond each opening. As every wave which comes up to the 

 obstacle is split into as many little waves as there are openings, 

 this light will exhibit interference phenomena if it fall on a screen, 

 and the screen will exhibit alternate lines of light and darkness. 



Let 1, 2, and 3, Fig. 11, be three successive openings ; let a d, 

 b d, be two rays falling together on the screen ; the point d will be 

 light if the waves strengthen each other, as when the path a d = 

 bd. 



They will annul each other when the path ad is half a wave 

 longer or shorter than b d, and the screen will be dark. 



All minute objects, when illuminated from below, even those 

 measuring T ^^ or more of an inch in diameter, produce diffraction 

 and give rise to interference, which results in more or less modified 

 images. Diatoms are more especially liable to produce marked 



