ON LIGHT. 319 



kind should, in the case of light, be carried still farther 

 when we consider that the aerial impulses by which 

 scftttid is propagated, take place in the direction of the 

 sound-ray, so that in passing (for instance) through an 

 aperture in a screen, a quantity of air is pushed bodily 

 .through it, and issuing on the other side, causes an in- 

 crease of local density due to the actual introduction of 

 additional air at a given spot, which of course tends to 

 .expand laterally as well as to push forward, and is not re- 

 strained from so doing by the lateral pressure of the 

 rest of the wave, which is suppressed. Light, as we have 

 already intimated, is propagated through an elastic 

 medium more in analogy with a solid than a fluid, 

 (which Newton's objection implies,) and by vibrational 

 movements not in the direction of the ray, but trans- 

 verse to it, so that in its passage through an aperture, 

 or beside the edge of an obstacle, this cause of lateral 

 spreading, at least, is absent; whatever other this 

 peculiar mode of propagation may call into action. 

 Lastly, however, the phenomena of diffraction with 

 which we are now concerned rely for their explanation 

 on this very principle that shadows are not strictly 

 definite, and that there really is a certain, and not very 

 small amount of lateral spreading of the light into the 

 space occupied by what may be called the geometrical 

 shadow. 



(103.) If a room be darkened and the sun allowed to 

 shine into it only through a very small aperture, as a 

 pin-hole, the rays which emanate from different points 

 of its apparent disc, passing srraight through and cross- 



