374 THE NATURE OF LIGHT. 



same side of the sun and in a straight line with the sun (conjunc- 

 tion), and farthest from each other when they are on opposite 

 sides of the sun and in a straight line with that luminary (opposi- 



FIG. 276. 



tion). Hence, Roemer argued that it requires 16 min. 36 sec. for 

 light to pass over the diameter of the earth's orbit, from Eio E'. 

 This distance being approximately known, the velocity of light is 

 easily computed. 



(&.) The velocity of light has been measured by other means, 

 giving results that agree substantially with the result above given. 

 When astronomers accurately determine the mean distance of the 

 earth from the sun, the velocity of light will be accurately known. 



(c.) It would require more than 17 years for a cannon-ball to pass 

 over the distance between the sun and the earth ; light makes the 

 journey in 8 min. 18 sec. For the swiftest bird to pass around the 

 earth would require three weeks of continual flight ; light goes as 

 far in less than one seventh of a second. For terrestrial distances, 

 the passage of light is practically instantaneous ( 435). 



589. Effect of Distance upon Intensity. 



The intensity of light received ~by an illuminated 

 body varies inversely as the square of its distance 

 from the source of light. 



(a.) Let a candle at 8 be the source of light ; A, a screen one foot 

 square and a yard from S ; B, a screen two feet square two yards 

 from 8', C, & screen three feet square three yards from S. It 

 will easily be seen that A will cut off all the light from B and (7. 

 If now A be removed, the quantity of light which it received, no 

 more and no less, will fall upon B. If now B be removed, the 

 quantity of light which previously illuminated A and B will fall 

 upon (7. We thus see the same number of rays successively illu- 



