PRECISION TOOL MAKING 401 



space formed between the upper flat and the top surface of the gage 

 block. In accordance with the theory of interference, the distance 

 between the flat and the gage at the first band adjacent to the edge 

 where contact between the two surfaces is made, is one half the wave 

 length of the light used, which if red would be .0000125 in. The 

 distance at the second band is then one wave length or .000025 in. If 

 there are a total of three bands, the distance at the edge of the block 



Fig. 27 — Metal flat, optical glass flat, and gage block in position for measuring 

 .375" plug by light wave interference method 



is .000037 in. or the difference in size between the plug and the gage is 

 .000074 in., since the distance between them is twice the width of the 

 block. The interference method gives a reliable and permanent unit 

 of measurement and is probably one of the greatest refinements in 

 precision measuring. In addition to measuring lengths, it can be 

 used for checking the accuracy of flat surfaces, tapers, etc. 



For more precise comparisons of gages and for the direct measure- 

 ments of gage blocks in terms of wave lengths of light there is available 

 a special form of the Michelson interferometer made by Zeiss, having 

 a monochromator for selecting the particular wave length to be used. 

 Fig. 28. This instrument is a comparatively recent development and to 



