544 



Transactions of the Society. 



impossible to determine when the two wires are exactly super- 

 imposed, I make from time to time a number of readings on each 

 side of the fixed wire to determine the zero error, which in my 

 Zentmayer filar is at present one-half division (minus). In measur- 

 ing an object such as a blood corpuscle, I enclose it between the 

 inner margins of the two wires ; hence the diameter of one wire (or 

 half the sum of the diameters of the two wires, which is the same 

 tiling) must be allowed for : this is equal to two divisions." So 

 that the total correction in such case would be — 2 ■ 5 divisions. 



In making measurements upon a stage micrometer, the wires 

 are both placed parallel to the two defining lines of the stage 

 micrometer, and on the left side thereof, respectively. Where these 

 defining lines are both of the same thickness, all that is now neces- 

 sary is to apply the correction for zero error to the reading. If, 

 however, the defining lines are of different thicknesses, in order to 

 make the two methods used in this work comparable, it is necessary 

 to measure from centre to centre of the defining lines. An equal 

 number of readings are, therefore, made on both sides of the two 

 defining lines, and the mean of the two sets of readings taken, which 

 eliminates the effect of unequal thickness of the two defining lines, 

 and gives the reading from centre to centre. 



By Mr. Keeley's method, it will be observed that two readings 

 are made, one on each side of each defining line, and the mean 

 adopted as the final reading. This method is more laborious, but 

 the recorded results show that it yields a very high degree of 

 accuracy. 



In order to give some idea of the precision of the results ob- 

 tained, from ten to twenty readings are given in each series, with 

 the resulting probable error, computed in the usual manner. 



Using a Spencer stand (new model), Gaertner filar micrometer, 

 with a Leitz No. 3 objective, the following readings were obtained 

 on the first T ^ mm. of the micrometer under observation. One 

 division = 0*2073 /x. 



