DEVICES FOR SEEING . 307 



in the sand, the men entering it are slowed up while the men 

 still walking on the hard surface can keep their regular pace. 

 The direction of the march will be changed, the line wheeling 

 right somewhat. The same thing happens when the line emerges, 

 since those men at the left of the line get out of the sand while 

 those at the right are still plodding through it. This rough 

 analogy may help beginners to clarify the process of refraction. 

 The stepping of the men corresponds to the vibration of the 

 particles in the formation of the wave of light. 



Now white light is a blend of many-colored lights, each with 

 its own specific rate of wave-motion. The violet waves are 

 short waves, the red are long, and the intermediate colors, 

 indigo, blue, green, yellow, orange, have increasing wave-lengths. 

 Only the primary colors are here mentioned; there are innumer- 

 able intergrading shades each of which has its own length of 

 wave. When a beam of such white light traveling in air passes 

 through a glass prism, it emerges spread out into a band of 

 color. The analogy of the marching men may again help to 

 give some notion of why this occurs. Suppose a company is 

 marching eight abreast. The first line is made up of short men 

 who naturally take short steps, the next of taller men who step 

 less often, the third line of still taller men whose steps are still 

 longer, and so on. (This is a very unmilitary supposition, 

 but these men are an illustration, not troops.) Again the 

 company is tramping through the wedge-shaped area of sand. 

 The short- stepping men will be retarded in it more than those 

 who take long steps because they must step in it more frequently. 

 When the company emerges, therefore, the line of very tall men 

 will be bent out of its original course least, the line of the very 

 short men most, and the intermediate lines will fall between 

 these. The analogy is very crude but it may help to visualize 

 this process of dispersion of light. The men who take long steps 

 correspond to the long light waves like those of red light, while 

 the men who take short steps are analogous to the short waves 

 of such light, as the violet. 



