248 B. A. Gould — Address delivered he/ore 



During the past year Dr. Guillaume, assistant in the Bureau, 

 has investigated the same question, availing himself of inde- 

 pendent data. Using the results obtained last year by Dr. 

 Benoit, according to which, at the temperature 16° -25 C, 



mm 



BessePs toise = 1949 061 

 Toise No. 10 = 1949-060 



and accepting the determination of Clarke 



Toise No. 10 = 2-1315091 yards 



we have the result 



l m =z 39-3699 in. 



which differs from our Mr. Tittmann's previous result by only 

 one ten -thousandth part of an inch. 



The relation of the British measure of length to the meter 

 seems thus very closely determined ; but the same is now to 

 be still further examined by a direct comparison at the Inter- 

 national Bureau of the two principal standard yards, which 

 will be taken to Breteuil for the purpose by the Warden of 

 the Standards. 



At our annual meeting a year ago, I had the pleasure of an- 

 nouncing that arrangements had been essentially completed 

 for the determination of the meter in lengths of light-waves 

 corresponding to known and easily reproducible rays of the 

 spectrum. In spite of some serious difficulties this plan was 

 successfully carried out. Its execution was confided to Prof. 

 Michelson, who was granted a six months' leave of absence for 

 the purpose by Clark University. Through his exertions the 

 apparatus was completed in time for him to take it to Paris in 

 the first week of July last, and he has been assiduously occu- 

 pied with the measurements. Although these are not yet 

 absolutely completed, only a few more weeks of labor will 

 probably be necessary to conclude the investigation, which has 

 been attended with the most gratifying success. Prof. Michel- 

 son discovered during the last spring, that the metal cadmium 

 afforded three rays, sufficiently monochromatic for the pur- 

 poses, and situated respectively in the red, the green and the 

 blue. The number of wave-lengths to the meter for each of 

 these will soon be fixed with extreme accuracy, and the meter 

 thus brought into relation with natural units ; so that, were 

 every metric prototype in existence to be lost or destroyed, 

 it would still be possible for the meter to be restored by future 

 generations, with accuracy to less than half a micron. 



