Radiant Forces. 137 



arrive at his distant surface, which deviated infinitesimally from 

 true parallelism at the time of starting. 



By drawing two diverging lines on a piece of paper, it is 

 easy to see that at certain distances they can only reach bodies 

 of proportionate dimensions. Thus, if at six inches from their 

 point of starting, two diverging lines exactly touch the top and 

 bottom of a perpendicular line one inch long, it will be found 

 at six inches further, or one foot from their starting point, they 

 will exactly touch the top and bottom of a line two inches 

 long. 



If it be desired to make these matters plain to young 

 people, it will be better to construct a model than to draw a 

 diagram. This may be easily done by fastening a pin to a piece 

 of wood or cork, and carrying from it two lines of thread, to 

 the top and bottom of a perpendicular line running through the 

 centre of a piece of card two inches square, and one foot 

 removed from the pin. A second piece of card, one inch 

 square, should be placed six inches nearer, and the threads 

 will exactly pass over the upper and lower surfaces of the 

 two cards. Two other threads should then be extended so as 

 to touch the cards at positions exactly at right angles to the 

 former. It then becomes evident to the eye that a card one 

 inch square, at six inches from the pin, which represents the 

 source of light, can receive as many diverging rays as can be 

 caught by a two-inch square at double the distance. But a 

 two-inch square has a surface equal to four one-inch squares 

 (as may be readily demonstrated by experiment*) , and therefore, 

 a portion of the two-inch square, equal to a one-inch square, 

 could only receive one quarter as much light as the one-inch 

 square could receive, through being at only half the distance 

 of the former one from the source of light. 



This model, or a diagram, will show, that for diverging rays 

 to reach an object of moderate dimensions, like the two-inch 

 card, a little way off the source from which they radiate, it 

 is necessary that they should diverge at a very gentle rate. 

 Bays that diverge rapidly can only be caught by a moderate 

 sized object when it is placed near to the point from which 

 they proceed, and even so large an object as our earth can 

 only receive from the sun rays that for all practical purposes 

 must be regarded as parallel, though a large number of them 

 reach us with an infinitesimal divergence. 



Astronomers calculate the quantity of light and heat that 

 can be received from the sun by the different planets revolving 

 round him, by estimating their distances from the central orb. 



* In teaching young folks, or uninstructed adults, those things should be 

 shown by cutting pieces of card, and placing four one-inch squares on one two- 

 inch square. 



