254 



IMPORTANCE OF OUR EYES 



should we abuse these delicate organs we call the eyes? 

 The daytime is not long enough for us, so we provide 

 ourselves with artificial light and keep our eyes at work 

 part of the night. We read fine print in dim light, and 

 injure our eyes in a hundred ways. Our eyes are of so 

 much importance to us that it seems wise to devote an 

 entire chapter to them and their care. 



Lenses and what they do. We have all tried to set fire 

 to a pile of leaves or paper with a burning glass. An 

 ordinary reading glass found sometimes on the library 

 table is a common example of a simple lens. If we look 

 at it in cross section, we find that it bulges outward in the 

 middle in both directions. We call such a lens double con- 

 vex. If we hold such a lens so that the sun shines through 



it, and move a piece of 

 paper toward or away 

 from it, one place will 

 be found where only a 

 tiny spot will be lighted, 

 but this will be ex- 

 tremely bright, and 

 enough heat may be 

 concentrated to set the 

 paper on fire. This 

 point at which the sun's 

 rays have been brought 

 together (see F in fig- 

 ure) is called the focus. 

 When light passes 

 through the air to the glass, it moves in parallel rays, but 

 the glass bends the rays so that they come together at 

 one point. This point is called the principal focus of the 



Which lens has the shortest and which the 

 longest focal length ? 



