1. Arrangement of the comparator. 



A complete description of the comparalor can be found in Dr. J. 

 A. C. OuDKMANs, "Die Triangnlation von Java, erste Abteiling" ; we 

 may therefore confine ourselves liere to a siiort account of tiie 

 arrangement. 



A wooden case over four metres long inside, contains a long iron 

 carrier, which can be moved upon rails from one side of the case to the 

 other, at right angles to the length. Upon this carrier the measuring 

 bar and the metre wiih which it is to be compared, are placed 

 parallel to each other, while the metre with the box in which it 

 is placed can be moved along the carrier in the direction of its 

 length, and can so in turn be placed opposite to each of the four 

 parts a, b^ c. and (/ of the measuring rod. 



The w^ooden case is further provided with a strong iron frame 

 to which the micrometer-microscopes are attached at a distance of 

 exactly a metre and which can be moved upon rails, independent!}' 

 of the carrier, above the measurijig bar and the metre. 



The first thing (o do is to place the metre opposite the first part 

 of the measuring bar, parallel to and at the same height as the 

 bar, and to push the carrier upon which they both lie as far as 

 possible sideways across the case, until it touches a pair of correc- 

 tion screws. If evci-ything is pi'operly arranged, the microscope 

 frame upon its rails can then be placed so, that the two microscopes 

 are just above the end lines of the metre, or the part a of the 

 measuring bar. If the carrier is ihen moved to I he other side of 

 the case, where it similarly touches two screws, the microscopes 

 will be directed just above the end lines of part a or of the metre. 



By focussing with the micrometers accurately upon the end lines 

 of the metre and of part a in both these positions, it is easy to 

 find the difference in length between them, expressed in micro- 

 meter-divisions. By subsequently placing the metre successively opposite 

 to the portions b, c. and jl of the measuring bai'. and making the 

 same observations, the data are procured, by which the length ot 

 the measuring bar can he determined in metres. 



In order to be certain that in the successive measurements of 

 portions a, b, c, and d the microscopes were each time directed 

 upon the same points of the division lines, small brass plates 

 provided with a point in the middle were fixed on the middle of the 

 bar near the division lines, in this way fixing a line along the middle 

 of the measuring bar. The correction screws at the sides of the case, 

 (against which the carrier moves up in its sideways movements) 

 were so adjusted, that these points came exactly under the fixed 



