STUDIES IN NATURE 



sun does not stop shining, even though it does 

 not shine on us. But if the sun is always shin- 

 ing, surely the heavens, across which it shines, 

 ought always to be bright ? We see the stars at 

 night, which are much farther away than the 

 sun, and as the sun shines always, on all sides 

 of itself, and not only just in that particular spot 

 of the heavens where our earth happens to be, 

 we surely ought to see the light from the sun 

 going out in all directions. We see the light 

 from a lamp or candle brightening other objects 

 in the room, even if we put a screen or hold a 

 book between the lamp and our eyes. But the 

 heavens are quite dark at night except for the 

 stars, and sometimes for the moon. If we think 

 about these two things a little longer, we shall 

 perceive that we can see different objects in two 

 ways. Either the objects themselves are send- 

 ing out light, as are the sun or a lamp, or else 

 the light from such a source is falling on them 

 and then reaching our eyes. Again, we find that 

 if a screen be held between us and any object 

 we cannot see the object. From this we con- 

 clude that the light which previously made it 

 visible travelled in straight lines. A luminous 

 source is only visible when there is an unin- 

 terrupted straight line from it to our eyes, and 

 we can only see another object when the way is 

 clear between it and ourselves and light shines 

 on it. 



Light has different colours ; the light of a 



(52) 



