162 MINERALOGY 



the arrow at y, when at that instant another crest is passing the 

 point y. Each time that a completes the path between x and y, 

 and returns to its original position, moving in the same direction, 

 an entire crest and trough have passed, or one wave length, denoted 

 by A. The wave length is the distance oo, measured between 

 the paths of the two particles aa, occupying the same position in 

 regard to the arrow and traveling in the same direction; such 

 particles are said to be of like phase. The period is the time that 

 is taken by any particle to complete the swing back and forth and 

 to return to its original position and condition. The amplitude 

 is the distance oy = ox from the median line to the highest point 

 in its path. It is also to be noted that the particle a moving 

 back and forth along the path yx is not carried forward along 

 the arrow, but like a block of wood rises and falls on the waves. 

 A wave front is formed by the particles or points which are in- 

 fluenced simultaneously as each wave passes them; they are all 

 in the same phase and form a surface or line at right angles to 

 the direction of transmission, at any particular point. When the 

 wave surface is a curved surface, the plane tangent at any particular 

 point will be at right angles to the ray or the direction of transmis- 

 sion at that point. All the above terms are equally applicable to 

 the light wave, but just what the change of conditions along the 

 path xy is, is still somewhat in doubt. 



The intensity of light is proportional to the square of the amplitude, 

 and the color will depend upon the length or more correctly on the 

 periodicity or number of vibrations per second of the wave. Deep 

 violet light at one end of the spectrum has a wave length of .000396 

 mm., while dark red at the other end has a wave length of .000795 

 mm., or about double that of violet, and the yellow sodium light 

 is about halfway between these two, or .00059 mm. Vibrations 

 larger or smaller than these the eye is unable to detect as light ; 

 but that heat rays do exist above and actinic rays below is easily 

 demonstrated by detectors other than the eye. 



Light waves of all lengths travel in a vacuum with the same 

 velocity, but they differ in their period, since short waves, as violet, 

 must vibrate twice as quickly as the red waves, which are double 

 their length. Upon entering a transparent medium the velocity 

 of light of all wave lengths is modified ; the extent of this modifica- 

 tion will depend both upon the medium through which the light is 

 traveling and upon the wave length or color of the light ; so that 

 lights of different wave lengths will vary in their velocities upon 



