POWER OF SHORT SPACES. 29 



are the rays which fall from it upon the object ; or the more 

 nearly do they approach to being parallel. " From a (fig. 2) 

 there is much divergence, from b less, from c less still, and 



rays from a greater distance, as those represented by d and e, 

 appear parallel. If the distance of the radiant point be very 

 great, they really are so nearly parallel, that a very nice test is 

 required to detect the deviation. Rays, for instance, coming 

 to the earth from the sun, do not diverge the millionth of an 

 inch in a thousand miles. Hence when we wish to make ex- 

 periments with parallel rays, we take those of the sun."* 

 When such rays therefore are intercepted by an opaque body, 

 the breadth of the shadow, for we are not now speaking of its 

 length, is equal to that of the substance. The student in per- 

 spective, is aware of this fact, and the fine effect of a good 

 landscape painting is to be referred in part to the strictness 

 with which this relation is observed by the artist. 



If the rays are divergent, as when the light-emitting body is 

 very small, a mere point, the shadow is larger than the object. 

 Thus if L (fig. 1) be the luminous body, and O the obstacle, 

 the circular figure, S, on the screen, W, being a cross section 

 of a shadow which is continually increasing in breadth, is 

 larger than the object O. "The shadow of a hand held be- 

 tween a candle and the wall is gigantic ; and a small paste- 

 board figure of a man held in a divergent pencil of light, and 

 near its source, throws a shadow as big as a real man. The 

 latter fact has been amusingly illustrated by the art of making 

 phantasmagoric shadows." Divergent pencils are easily pro- 

 cured from a pin-hole, a taper, a street lamp, a carriage lamp, 

 &c. 



When a convergent pencil of rays is obstructed by an opaque 

 body, the shadow is smaller than the object, and if not re- 

 ceived on a screen, would taper to a mere point. This is true 

 of the shadows of all the planets, and of the earth, because 

 they are less than the sun. It is exemplified when the moon 

 * See Arnott's ' Elements of Physics.' 



