46 gray: micrometer microscopes 



never before been investigated. A curious feature of this micro- 

 scope was the regular repetition *of this series of corrections every 

 other turn, with a somewhat different but equally regular series 

 for the alternate turns. Column C represents a screw of unusual 

 excellence, made by a firm noted for high-grade astronomical in- 

 struments. D represents the average and E the worst of a group 

 of ten others made by this same firm. The irregularities which 

 these ten exhibit are of the same order of magnitude as those of the 

 best microscopes belonging to the International Bureau of Weights 

 and Measures. 



While the need of applying corrections to some of these micro- 

 scopes is perfectly evident, it may be well to point out that screw 

 irregularities are of importance even in such micrometers as those 

 of the International Bureau of Weights and Measures. An exami- 

 nation of the correction tables appended to one of Benoit's 

 papers^ shows that an error of about 1.5 ^ can be made in com- 

 paring two lengths if the only corrections applied are those for 

 deviations of the mean screw values from the nominal value of 

 100 M per revolution; and further that most of the error will be due 

 to neglecting the periodic corrections. 



In comparing two lengths with a transverse comparator it is a 

 common practice to multiply the changes in each ixiicrometer 

 reading by the mean scale-value and subtract. But even less labor 

 is required to take into account all necessary corrections, after the 

 microscopes have been calibrated once for all. The writer's 

 method of doing this is essentially a simplification of the procedure 

 followed by the International Bureau of Weights and Measures 

 in measuring the national prototype meters.- The principal gain 

 is effected by reducing to two tables (both of which can be mount- 

 ed on a single card) all the information contained in the six tables 

 used by the International Bureau. The table for the left micro- 

 scope gives at one inspection the total correction AL that must be 

 added to any reading U of the left microscope to give the cor- 

 rected reading aL; that for the right microscope gives — A^, the 



' J. R. Benoit: Mesures de dilatation et comparaisons des regies metriques, 

 Trav. et Mem., 2, C. 131, 1883. The tables are on pp. c, xi, and xii. 

 ^ Described in the above mentioned paper by Benoit. 



