372 THE NATURE OF LIGHT. 



it toward a candle flame. The inverted image may be seen upon 

 the oiled paper. Th size of the image will depend upon the dis- 

 tance of the flame from the aperture. The apparatus rudely repre- 

 sents the eye, the pin-hole corresponding to the pupil and the oiled 

 paper to the retina. (Almost any housekeeper will give you an 

 empty tin can. Place it upon a hot stove just long enough to melt 

 off one end, thrust a stout nail through the centre of the other 

 end, cover the nail hole with tinfoil, and you will have the greater 

 part of the apparatus.) 



586. Shadows. Since rays of light are straight, 

 opaque bodies cast shadows. A shadow is the dark- 

 ened space behind an opaque body from which all 

 rays of light are cut off. It is sometimes called the 

 perfect shadow or the umbra. If the source of light be a 

 point, the shadow will be well defined ; if it be a surface, 

 the shadow will be surrounded by an imperfect shadow 

 called a penumbra. The penumbra is the darkened 

 space behind an opaque body from which some of the 

 rays (the rays from a part of the luminous surface) are 

 cut off. 



FIG. 274. 



(a.) Hold a lead pencil between the flame of an ordinary lamp and 

 a sheet of paper held about two feet (61 cm.) from the lamp : (1.) 

 When the edge of the flame is toward the pencil ; (2.) When the 

 side of the flame is toward the pencil. 



