S04 



Each of the three observers made two of these observation-series 

 in succession. 



After these six series, the metre was turned round in its case, so 

 that the marlv wliich first showed on the left hand side now lay 

 on the right hand side ; and in the same way as at the beginning 

 of the measurements, the position of the metre with regard to the 

 measuring bar and the microscopes was then properly regulated. As 

 the comparator case had to be opened for this, there was a disturb- 

 ance in the equilibrium of the temperature. An hour or IV2 was 

 therefore allowed to pass before fresh measurements were begun. 

 As in the first position, each of the three observers then took two 

 series of observ^ations in this second position. 



For the determination of the temperature in the comparator, at 

 the beginniug and at the end of the 6 series with the same position 

 of the metre, the thermometers E, F, G. and H were read. These 

 readings served only to ascertain, whether disturbances of tempera- 

 ture had occurred in the comparator. In none of the series which were 

 used for the determination of the length of the measuring bar was 

 this the case; there was therefore no further use made of the 

 readings of the thermometers E, F, G, and H, any more than of 

 the records of the registering thermometer. 



As in the computation of the results the differences of the readings 

 of the right-hand and left-hand microscopes are used, the influence 

 of a personal error of adjustment will disappear from the results, 

 if both end lines, the micrometer wires and the optic images in the 

 two microscopes arc exactly alike. This complete equality however 

 does not exist. The lines are, as far as can be seen, all equally fine 

 and faultless, but the distance of the micrometer wires is smaller 

 in the one microscope than in the other, so that the appearance of 

 the line, when it is placed between the two micrometer wires, is 

 different in the two microscopes. In order to eliminate the personal 

 error arising from this, the observations would have to be repeated 

 after exchanging the microscopes, or else with the mici'oscopes in 

 the same position, but the observer standing the second time on the 

 other side of the comparator, so that the microscope which was first 

 on his right hand, is now on his left. 



The latter method is simpler than the former, and had the further 

 advantage (over the changing of the microscopes) that the observer, 

 who first stood nearest to the measuring bar, is now nearest to the 

 metre, and an irregular influence of the heat radiated by the observer 

 will thus be also, at least partially, eliminated. 



On these grounds the observers, after they had compared each of 



