324 PROCEEDINGS OP THE AMERICAN ACADEMY 



cal plane with the contacts of the stops at one end, and the micrometer 

 line of M must be placed in the same vertical plane with the contacts 

 of the stops at the other end. This is accomplished in the following 

 manner. The stop-plate is thrown back by means of the controlling 

 lever, and a rectangular block of metal is placed between the stop and 

 the defining surface of the bar. This block has two of its faces exactly 

 parallel and one inch apart. The upper surface is supposed to be in 

 the same plane as the upper surface of the short graduated reference- 

 bar attached to the stop-plates. The ujDper surface has a slight pro- 

 jection at each end, which allows defining lines one inch apart. This 

 surface is bisected approximately by a horizontal and a perpendicular 

 defining line, and a perpendicular line is drawn at each end exactly 

 half an inch from the line at the middle point. 



The first step is to set the micrometer M^ for coincidence at the in- 

 tersection of the cross lines upon the plate. The block is then turned 

 180° and a second reading of M-^ is taken. One half of the difference 

 between the two readings will measure the distance of the defining line 

 on the block from the middle point between two stops when contacts 

 are made with the end surfaces. If the plate K is moved forward a 

 distance equal to 



the micrometer line of the microscope will be in the perpendicular 

 plane which passes through the point of contact formed when the two 

 stops are brought together by the removal of the block. 



In the same manner the reference line of micrometer M is brought 

 into coincidence with the plane passing through the point of contact of 

 the stops at the other end. The distance between the micrometer lines 

 of the two microscopes is now equal to the length of the end-measure 

 standard. 



(4.) The line-measure standard to be compared is now placed upon 

 the table S, and the distance between the defining lines of the meter 

 is compared with the distance indicated by a constant plus the read- 

 ings of the microscope micrometers. The adjustments required are the 

 same as those described under division («), page 302. 



The comparison of short-end measure gauges can be expeditiously 

 made by means of the graduated reference-bar attached to the stop- 

 plate. For this purpose the two lower slides are brought near enough 

 together to allow the stops of the upper plates to be brought into con- 

 tact. Both slides are then securely fastened to the cylindi-ical ways 

 upon which they rest, and one of the stop-slides is securely clamped 



