48 The Microscope. 



American Academy of Arts and Sciences, Professor Rogers visited 

 London and Paris, for the purpose of obtaining authorized copies of 

 the English and French standards of length; the Imperial yard, and 

 the Metre des Archives. The copies then obtained are the first 

 brought to the United States, and have since been used as the basis 

 of comparison for the bars made by him, which now serve as stand- 

 ards of length for Harvard, Yale, Columbia, Princeton and other 

 colleges; The United Stated Signal Service and the Lick Observatory; 

 also a combined yard and meter on one bar, constmcted for the 

 Department of Standards of the British Board of Trade, to be used 

 in the official determination of the relative lengths of the yard and 

 meter. 



The copy of the French meter is a copper bar, which was pre- 

 pared by Professor Tresca, of the Conservatoire des Arts et Metiers, 

 at Paris. It was transferred from meter No. 19, of the conservatory, 

 on February 6th, 1880, at two o'clock in the morning, and later its 

 absolute relation to the original was determined. This bar, now 

 known as the Tresca meter, served as the standard for making "Cen- 

 timeter A.," which was prepared by the U. S. bureau of weights 

 and measures, for a committee on micrometry, appointed by the 

 American Society of Microscopists, and adopted by that society as 

 its standard of length, at its annual meeting in 1883.* 



A comparator recently constructed from the drawings of Pro- 

 fessor Rogers, and now used by him in his laboratory at Water- 

 ville, possesses several advantages over the Rogers-Bond instrument, 

 and leaves little to be desired. He is at present using this compar- 

 ator in a series of experiments, undertaken for the purpose of 

 ascertaining the laws which govern the expansion and contraction 

 of different metals under variations of temperature, where their 

 mass is considered. To show the care taken to eliminate possible 

 sources of error in these observations, care and patience typical of 

 all his scientific work, it is only necessary to mention, that in order 

 to obtain the absolute corrections of one of his mercurial thermom- 

 eters, used in taking the temperature of a metal bar, he made 

 twenty-two thousand comparisons of its scale, with a known stand- 

 ard. 



It is his intention to establish at Colby University, a one hun- 

 dred foot standard of measurement, for the comparison and correc- 

 tion of surveyors' chains, tapes, lines, etc. When this is constructed, 

 it will be the longest standard of length for practical comparisons in 

 the world. 



• Proceeding-! Am. S )C. of Microscopists, ISS-J. p. 184. 



