STANDARDS OF MEASUREMENT — BRIGGS 167 



Bureau, the National Physical Laboratory, and the Physikalisch- 

 Technische Reichsanstalt. From all of these determinations, no 

 measurable change in dimension with time has been detected in any 

 of the quartz gages. The end surfaces were plane and parallel within 

 less than 0.02 micron and the maximum difference in various deter- 

 minations of the length of any one gage did not exceed 0.02 micron, 

 or 2 parts in 10 million. 



In making these standards the decimeter length was chosen 

 because this is about the maximum length for which clear interfer- 

 ence rings can be obtained with the sharpest spectral lines. With 

 these lines the number of waves in the path ranges from 300,000 to 

 400,000. The fractional order can be measured to about 0.01 of a 

 wave length, so it is possible to attain a precision of 1 part in 30 

 to 40 million in the comparison of two wave lengths and about 1 part 

 in 10 million in the direct determination of a material length 

 standard. 



STANDARD WAVE LENGTHS 



The internatianal primary standard of wave length is the red 

 radiation of cadmium. The wave length of this radiation as officially 

 adopted by the International Committee of Weights and Measures 

 is 6438.4696 angstroms under specified standard conditions. 



Tribute should be paid to the painstaking studies of Michelson 

 40 years ago that led him to select the red radiation of cadmium as 

 the standard wave length to be evaluated in terms of the meter. All 

 of the spectroscopic work which has been done since that time, 

 including the study of the spectra of helium, krypton, neon, and 

 other gases then unavailable to Michelson has failed to disclose 

 another strong line of superior quality. Two krypton lines are the 

 closest rivals for this honor. 



Using the cadmium line as a primary standard, the International 

 Astronomical Union has adopted a series of secondary wave-length 

 standards, including 20 neon lines, 20 krypton lines, and about 300 

 lines in the iron arc. The measurements of Meggers, Kiess, and 

 Humphreys at the National Bureau of Standards have contributed 

 substantially to the establishment of all of these secondary standards. 

 Most of the neon and krypton standards are known relative to 

 cadmium with a precision of 2 or 3 parts in 100 million, while the pre- 

 cision of the iron standards is about one order less. These standards 

 provide the framework of all spectroscopic measurements. 



THE STANDARD PLANE SURFACE 



In order to obtain a surface which is optically plane to a high 

 degi'ee of precision, it is necessary to prepare three surfaces, which 

 when tested in pairs in any of the three possible combinations and 



