Comparators for Measures of Length. 

 MEASURES OF MILLIMETER DIVISIONS. 



215 



One division=.ooo3i75 cm. 



It will be seen that the individual errors of graduation are 

 practicably insensible. It is not supposed that the figures 

 v^^hich represent millionths of an inch are reliable to the last 

 unit. They are given in the sixth decimal place in order to 

 make sure of the figure in the fifth place. 



It is now to be noted that the errors thus obtained are entirely 

 relative errors. They give no indication whatever of the absolute 

 value of any of the spaces measured. If the entire length of the 

 half inch is, e.g., .001 inch too long, to each of the corrections 

 given in the table, must be applied still further the correction, 

 .00020 inch. 



It is, therefore, necessary to make a careful investigation of 

 the entire length of the half inch and of the centimeter. 



This is done with a comparator adapted to the comparison 

 of spaces, ranging from coincidence to an entire yard or an 

 entire meter. Comparators of this class are usually con- 

 structed with two sliding plates, each carrying its own micro- 

 scope. A fundamental objection to this form is found in the fact 

 that the microscopes cannot be brought much nearer together 

 than three inches, by any direct means. For lack of space and 

 of illustrations, only a general description of the form which 

 the writer has had constructed, can be given here. It consists 



