308 



Mean 



Mean 



If the sum is talken of the lengths of the 4 portions of the 

 measuring-bar, we get for the lengtli of the whole measuring bar at 0°: 



ij. 

 Measuring bar = 4 X Metre 27 — 392.19 Wildeboer 

 ,, =r 4 X Metre 27—391.48 Dieperink 

 „ =r 4 X Metre 27—391.85 Bakhuyzen 



Mean for the three observers : 



Measuring bar = 4 x Metre 27-391^84. 



7. Mean errors. The errors in a series of observations are caused 

 by the pointing- and reading errors of the microscopes, the change 

 in the distance of the microscopes, erroneous determinations of 

 temperature and personal errors of observation. 



Owing to the excellent optical qualities of the microscopes and 

 the fine sharp end lines, the errors in the reading and pointing of 

 the microscopes are small. From the observations for the determination 

 of the periodic screw-errors we found for tiie mean reading-error, 

 from the mean of two observers, ± 0''%32 : this error leads to a 

 mean error of ± 0'',17 in one series of observations. 



The influence of other sources of error are difficult to determine 

 separately. We shall therefore try to calculate their combined effect, 

 in different ways, in order to find out, what systematic errors are 

 to be feared, and how the series of observations are to be combined 

 in order to obtain a result ni which the effect of the systematic 

 errors will be as far as possible eliminated. 



