16G ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 40 



of cadmium. The meter bars used in these measurements were 

 compared Avith the international prototype at the time they were 

 used. Any change in the length of the international prototype 

 would thus tend to be reflected in the derived value of the wave 

 length, in microns, of the red line of cadmium. Table 1 shows no 

 evidence of any systematic drift. 



It will be noted that the mean value of the determinations agrees 

 closely with the original value of Benoit, Fabry, and Perot 

 (6438.4696 angstroms), which since 1907 has been used by the Inter- 

 national Astronomical Union as the standard to which all spectro- 

 scopic wave-length measurements are referred. In view of this 

 fact, the National Bureau of Standards proposed, in the legislative 

 bill referred to above, that the meter and the inch should be given 

 supplemental definitions in terms of wave lengths of the red radia- 

 tion of cadmium, consistent with the relation adopted by the Inter- 

 national Astronomical Union. These supplemental definitions, if 

 adopted, will legalize the direct use of interference methods in the 

 jDrecise determination of the length of gage blocks and similar work- 

 ing standards. The practical value of this procedure cannot be too 

 strongly emphasized. 



END STANDARDS 



In the past few years the close tolerances placed on the mass pro- 

 duction of interchangeable parts has led to the extensive use of pre- 

 cision end gages. These are blocks, usually of metal, with two op- 

 posite faces accurately plane, parallel and a specified distance apart, 

 and are used to check various measuring instruments. The extent 

 to which these gages are used can be judged by the fact that more 

 than 50,000 have been tested at the National Bureau of Standards. 

 Using interference methods the surfaces of such gages can be tested 

 and the length determined with greater accuracy than by referring 

 to line standards. 



In order to provide the Bureau with end standards of the highest 

 precision, C. G. Peters and W. B. Emerson undertook the construc- 

 tion in 1934 of a series of end standards by direct interference meth- 

 ods, based upon the standard wave length of the red line of 

 cadmium. Fused quaitz was chosen for the blanks, because its low 

 expansivity (about one-thirtieth that of steel) removes the necessity 

 of accurate temperature control, and it can be given a high optical 

 polish free from imperfection. 



Fifteen blanks 2 cm. square in cross section and 10 cm. long were 

 cut from blocks of optically clear fused quartz. These were an- 

 nealed by heating to 1,150° C. and then ground and polished to size. 

 Extended measurements of these gages were carried out during a 

 period of 2 years, including measurements made by the International 



