THE NATURE OF LIGHT. 373 



(&.) Describe the shadow cast by a sphere and a luminous point. 

 By a sphere and a luminous globe of equal size. By a sphere and 

 a luminous globe of greater size, e. g., the earth's shadow. A solar 

 eclipse takes place whenever the eye of the observer is in the 

 shadow of the moon. Figure the shadow of the moon. Where 

 must the observer be to see a total eclipse of the sun ? To see an 

 ordinary eclipse (when the sun appears crescent-shaped)? To see 

 an annular eclipse ? 



587. Visual Angle. Tlie angle included be- 

 tween two rays of light coming from the extrem- 

 ities of an object to the centre of the eye is called 

 the visual angle. This angle measures the apparent 

 length of the line that subtends it. Any cause that 

 increases the visual angle of an object increases its appa- 

 rent size. Hence the effect of magnifying-glasses. From 



Fig. 275 we see that equal lines may subtend different 

 visual angles, or that different lines may subtend the same 

 angle. 



588. Velocity of Light. Light traverses the ether 

 with a velocity of about 186,000 miles or about 298 mil- 

 lion meters per second. This was first determined about 

 200 years ago by Koemer, a Danish astronomer. 



(.) At equal intervals of 42h. 28m. 36s., the nearest of Jupiter's 

 satellites passes within his shadow and is thus eclipsed. This phe- 

 nomenon would be seen from the earth at equal intervals if light 

 traveled instantaneously from planet to planet. Roemer found 

 that when the earth was farthest from Jupiter the eclipse was seen 

 16 min. 36 sec. later than when the earth was nearest Jupiter. But 

 Jupiter and the earth are nearest each other when they are on the 



