540 



Transactions of the Society. 



In order to compare these results to advantage, they are re- 

 peated in parallel columns : — 



The differences in these results are quite insensible. 



The instruments used by Mr. Keeley m measuring the two 

 T ^Q-mm. spaces of the first j-'g mm. were the same as above stated, 

 except that a Tolles -^-in. objective was used. With this combina- 

 tion, one division of the filar micrometer equals 0*1702 fi. Thirty 

 readings were made on each end of each space, with the following 

 results : — 



The same spaces were measured in 1907 by the writer, using 

 the Spencer stand, Zeiss filar, and the Leitz No. 3 objective before 

 described. With this combination, one division of the micrometer 

 equals 0*169/i. Ten readings were made on each end of each 

 space. 



Again, in 1909, using a Spencer stand, a Leitz No. 5 objective, 

 of 5 • 8 mm. focal length and • 77 N.A., and a Glaertner filar micro- 

 meter (pitch of screw, £ mm.), thirty readings more were made on 

 each end of each space. With this combination, one division of 

 the micrometer equals 0*067 /x. 



Combining these two series of ten and thirty readings by the 

 writer, we have the following results : — 



