50 R. S. Woodward — Iced Bar Base Apparatus. 



The observations were so arranged as to eliminate the effects 

 of uniform motions of the microscopes. Thus in the first 

 series of determinations the following program was adhered to : 



1. Measure of distance between end microscopes with meter, 



2. " " " " " with 5 m bar., 



3. " " " " " " meter; 



the second measure with the meter being made in a direction 

 opposite to that of the first. In later measures on this com- 

 parator the above program was supplemented by an initial 

 and final measure with the 5 m bar. 



The observations were made by two persons observing simul- 

 taneously at the respective ends of either bar. The observers 

 also exchanged positions in all cases to eliminate personal 

 equation. 



The determinations made in the manner just described 

 showed large ranges, amounting to l/200000th part at the 

 maximum ; and with the hope of overcoming the effects of 

 the irregular motions of the microscopes a different method 

 was tried with the same comparator. The essential feature of 

 this method consists in the use of an intermediary 5 m steelbar 

 subdivided into meter spaces. This bar, known as No. 18, is 

 similar in form to No. 17, except that it is cut down to the 

 mid depth at four intermediate places as well as at its ends. 

 The lines subdividing the bar into meter spaces are ruled on 

 platinum iridium plugs. This bar was mounted in the steel 

 Y-trough where its flexure could be controlled in the same 

 way as that of bar No. 17. The sub-spaces of No. 18 when 

 packed in melting ice were determined by direct comparisons 

 with Prototype Meter No. 21. Three series of six measures 

 each of the sub-spaces, and hence of the whole length of the 

 bar, were made. In the intervals between the first and second 

 and the second and third series, bars 17 and 18 were compared. 

 For this purpose No. 17 was mounted in an auxiliary wooden 

 trough similar to the Y-trough. Notwithstanding the di- 

 minished time-interval during which dependence on the sta- 

 bility of the microscopes was required by this process, the 

 results attained were still too erratic to give confidence. The 

 range in values for the metric sub-spaces of No. 18 rose to 

 l/200000th part and in the values for the whole length, or 

 sum of these spaces, to l/300000th part ; and these ranges 

 appeared to be directly referable to the large progressive, and 

 frequently large sudden, movements of the microscopes. 



Such being the unsatisfactory quality of the results attained 

 on the office comparator, it was determined in June, 1 892, to 

 build a new comparator designed more especially to meet the 



