ON fresnel's dikkractiox theory and babinets principle. 481 



To show the acting part of the wave surrounding the screening body 

 three conditions are required : — 



1. The plane of distinct vision must be made to coincide with the 

 edge of the diffracting body. 



" 2. The greatest portion of the diffracted rays must be preserved for 

 pi'oducing the phenomenon. 



3. The rays not employed in producing the phenomenon must be 

 cut off. 



It is not difficult to realise experimentally these three conditions, and 

 the result obtained is extremely brilliant with sunlight. Though much 

 less intense with the electric arc, it is nevertheless sufhciently distinct to 

 be projected. With a feeble source of light, such as an oil-lamp, the 

 phenomenon is still quite visible, but only by direct vision. 



Let us take a luminous source of circular form (the disc of the sun or 

 a circular hole in a diaphragm conveniently illuminated). Let the rays 

 diverging from this source fall upon an achromatic lens (0'5 to 1 metre 

 focal length) and produce at the conjugate focus a bright circular image 

 of the .source. Exactly at this focus let us place a black circular disc 

 having precisely the same diameter as this image. 



The eye placed behind this opaque disc will not see any point of the 

 luminous source, because all the rays are cut off by the disc ; it will 

 perceive, however, a luminous line round the edge of the lens. This is 

 in itself an illustration of the acting part of the Fresnel's wave, for the 

 edge of the lens is really a screen which intercepts the incident wave. 

 But the experiment becomes extremely striking if any object is put on 

 the surface of the lens : this object appears on a dark field as if surrounded 

 by a luminous line following even the smallest details of its edge. [In the 

 experiment projected before the Section the object shown was a branch 

 of fern.] 



Moreover, each particle of the unavoidable dust lying on the lens pro- 

 duces a luminous point, and for this reason the field is never quite dark. 

 Lycopodium thrown in front of the lens exaggerates this effect and 

 produces a sort of luminous nebula. 



The intensitv of the bright bordering line depends upon the accuracy 

 •with which the circular screen masks the focal image of the source ; if 

 the diameter of the screen is too small the field becomes luminou.s, if it is 

 too large the brilliancy of the line diminishes. If another shape be given 

 to the screen the luminous line is interrupted normally to the directions 

 in which the screpn extends much outside the image of the source. 



With the circular screen it is easy to observe the important particular 

 case to which Babinet called attention, viz., that a very thin opaque line 

 diffracts light exactly in the same manner as a transparent slit of the 

 same form and size. 



It suffices for this purpose to observe the shadows of successive wires, 

 straight or curved, of decreasing diameters. The dark space lying between 

 the two luminous edges in the case of a thick wire diminishes as the wire 

 becomes finer, and vanishes when its apparent angle becomes sufficiently 

 small ; then the wire appears on the dark field like the incandescent 

 filament of a glow-lamp. 



The slits corresponding to the wires are made with lines of decreasing 



breadths traced on smoked glass. The appearances are exactly the same 



as before, a double bright line being produced by a broad slit, a single 



bright line by a narrow slit, so that a fine transparent .slit and a fine 



' LS94. I I 



