﻿142 cox. 



With both the vertical and the horizontal apparatus the error of taking 

 the observations is reduced to a minimum, both because of the close range 

 of the microscope and the short displacement between the two bars to be 

 compared. ' There is practically no liability of disturbing the adjust- 

 ment while manipulating the microscope. A series of experimental com- 

 parisons were made and readings to a hundredth of a millimeter showed 

 no error in the vertical instrument. This apparatus is thoroughly 

 satisfactory for the testing of secondary standards. If it is desired to 

 compare with the greatest accuracy that has yet been obtained, then it 

 is recommended to follow the elaborate and more expensive plan of the 

 Ivaiserliche Normal-Aichungs-Kommission. 9 



9 Ztschr. f- InstruincntenJcunde (1895), 15, 313 and 353. 



