302 PROCEEDINGS OP THE AMERICAN ACADEMY 



the reference plane described by the movement of the microscope 

 carriage is nearly invariable. 



The third requirement is met by the following arrangement. 



The standards to be compared are placed upon the table S^ Fig. 1. 

 The whole framework to which this table is attached can be raised 

 or lowered by the hand-wheel C. It can also be moved forward and 

 backward, upon the ways shown in Figs. 3 and 4, by means of 

 bevelled gears driven by the hand-wheels beneath the bed-plate. 

 These coarse movements are designed to facilitate placing the stan- 

 dards to be measured approximately in the proper position in relation 

 to the microscopes M, M^. 



The carriage S is moved rapidly upon the V-shaped ways at the 

 upper surface of E, by a rack and free pinion movement. The rack 

 is shown at the left of the figure, while the handle of the pinion is 

 shown at D. 



The standards having been placed in such a position upon the table 

 S that the defining lines at one end shall be nearly over the fulcrum 

 below S^ the coarse motion in a direction at right angles to the motion 

 of the carriage S is obtained by the gibbed slides, S^ 



The slow movement of the carriage is obtained as follows : — 

 (1.) In translation, by the lever whose handle is shown at S^. 

 (2.) At right angles to the motion of translation, by the lever S^ 

 (3.) Vertically, by means of the screw S^ shown in the left-hand car- 

 riage. 



In practice it is found that these levers give a more delicate adjust- 

 ment than can be obtained with a screw movement, and quite as good 

 as can be obtained with the micrometer screw of the microscope. 

 It will be observed that the position of the standards with reference 

 to "the table S is not disturbed by any of these adjustments. The 

 time required in order to bring the terminal lines of a yard or meter 

 into proper position, adjustment, and focus under the microscopes, 

 is about forty seconds. 



Under the fourth requirement the following methods of comparison 

 are described. 



(a.) Comparison hy means of two Fixed Microscopes. 



The method of procedure will be illustrated by a description in 

 detail of the comparison of the standards of which the results are 

 given on the following pages. The following are the various steps of 

 the operation : — 



