414 SHADOW. 



rock-salt, rock-crystal, the diamond, and several other 

 crystallised minerals, glass, &c. Others are translucent 

 that is, they transmit light, but not without disturbing 

 sensibly the direction in which the light proceeds, and 

 hence objects cannot be distinguished when seen through 

 them ; ground glass, oiled paper, woven fabrics, belong 

 to this class of bodies. Most solid bodies, however, 

 unless they are in the form of a very thin layer, do not 

 permit light to pass through them ; they are opaque. 



When light falls in one direction upon an opaque 

 body, a portion of the space immediately behind the 

 body will remain dark. This portion of space is called 

 the shadow. The form which the shadow assumes in 

 each case depends on the shape and extent of the lu- 

 minous body as well as of that which causes the shadow, 

 and it depends also on the distance between the two 

 bodies. In empty space and in air, light is propagated 

 strictly in straight lines, and from this fundamental 

 fact the form of the shadow may be deduced in every 

 case. 



Let L in fig. 239 represent the flame of a common 

 paraffin lamp, and let K^ K^ K^ three discs of card- 

 board attached to a knitting needle, be the bodies which 

 cause the shadows /Si, $ 2 , S$, respectively, which are 

 received on a white rectangular screen. 



The ray of light which is emitted by a, the highest 

 point of the flame, and passes close to the highest point 

 of the body K 2 , reaches the screen at e, while the ray 

 which issues from the lowest point of the flame, and 

 passes close to the lowest point of K^ reaches the screen 

 at /, and obviously no ray of light can reach the space 

 intermediate between e and /. This space is the c true 



