PHENOMENA OF LIGHT 



175 



(a) Shadows cast by both Earth and Jupiter, and by the satellites 

 of both planets, with positions such that eclipses of the moons are 

 about to occur in both cases. 



(6) The earth's moon in such a position that an eclipse of the sun is 

 about to occur. 



FIG. 77. The velocity of light was calculated from observations made by 

 astronomers of the time-intervals between eclipses of the inner satellite 

 of Jupiter. 



10. Astronomers tell us that the innermost of the moons 

 of Jupiter is eclipsed at every revolution. Tell several 

 astronomical conditions implied in this statement. 



IMAGES BY REFLECTION 



1. Hold a small looking-glass in the sunlight at a window 

 and state the effect (a) on the direction of light as it comes 

 from the sun ; (6) on the degree of illumination where this 

 reflected light now falls. Distinguish between incident rays 

 and reflected rays. 



2. Hold a mirror so that the reflected light of a candle 

 flame enters the eye. What is seen in the mirror? Call it 

 an image of the flame. Along which rays of light is this to 

 be seen ? Why is it seen in this particular direction and not 

 in some other? 



