f,.",X 



Transactions of the Society. 



with which one division of the filar micrometer equalled 0*0256 p,. 

 The results given below are the mean of six (6) series of measure- 

 ments by the writer, and of an unknown number made by Pro- 

 fessor Pearce, of the U.S. Coast and Geodetic Survey, in 1882 — 

 with what instruments is to the writer unknown. It is perhaps 

 unnecessary to add that none of these measurements were made 

 with a view to comparison of the results. Micrometer measured, 

 " Centimeter A, 1882." 



The agreement between these two series of independent measure- 

 ments is close, the greatest difference being only • 1 //, or • 05 //. 

 from the mean of the two. The writer there stated that the use of 

 high powers in micrometry does not give so great an advantage as 

 had been claimed by some writers, and that clearness of definition 

 is of more importance than high amplification. The results of the 

 measurements here recorded confirm the writer in that opinion. 

 Neither is high angular aperture of great importance, at least not 

 in the comparison of micrometers. 



The measurements made by Mr. Keeley and the writer were 

 made on a Powell and Lealand glass stage-micrometer, being the 

 same scale described as " Powell and Lealand No. VIII." in a paper 

 by the writer published in this Journal for 1908, pp. 68t> and 689. 

 The lines of this scale are fine, well filled, and sealed in with 

 balsam and a cover-glass, and are reasonably well adapted to the 

 making of accurate measurements, with transmitted light — which 

 was used throughout. Mr. Keeley used artificial light and the 

 writer daylight. In it one millimetre is divided into tenths, and 

 the first tenth into hundredths of a millimetre. 



In order to avoid anv insensible bias, the figures recorded in 

 the said paper were not consulted until after all the other measure- 

 ments had been made and recorded. 



The instruments used by Mr. Keeley in measuring the 0*1 mm. 

 spaces were a Zentmayer Centennial Stand (which is an instrument 

 of very great weight and unusual stability), a Zentmayer filar 



