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BELL SYSTEM TECHNICAL JOURNAL 



Two of the characteristics of light are the constancy of its wave 

 length for a given color and its property of interference, which together 

 establish the fact that each interference band produced by the reflection 

 of light, as shown in Fig. 26, represents a very small definite separation 



Fig. 26 — Interference bands produced by the reflection of light waves 



between the surfaces producing the reflections. As this separation or 

 distance is a function of the wave length of the light used, the applica- 

 tion of this principle permits extremely accurate measurements to 

 within a few millionths of an inch. 



Fig. 27 illustrates the application of this method in measuring a 

 .375 in, plug, the equipment used being an optical glass flat, a metal 

 flat on which the parts rest, a .375 in. master gage block, and a mono- 

 chromatic light, usually red or green, having wave lengths of .000025 in. 

 and .000020 in. respectively. In order to simplify calculations, the 

 plug is placed so that its center is a distance from the gage block 

 equal to the width of the latter. Unless the gage and the plug are 

 exactly the same size, dark interference bands will appear across the 

 gage block when the light falls upon it, due to the wedge-shaped air 



