PROCEEDINGS OF THE SOCIETY. 267 



A paper by Mr. A. A. C. E. Merlin, " On a German-Silver Portable 

 Microscope made by Powell in 1850," was taken as read — the instrument 

 in question not having been sent to the Meeting. 



Mr. E. M. Nelson's paper, " On the Measurement of very Minute 

 Microscopic Objects," was read by Mr. J. W. Gordon. 



Dr. Spitta said, did Mr. Nelson make a definite statement in his paper 

 as to what were the objects which could be seen, but were too minute 

 to be photographed — did he give any instance ? 



The thanks of the Society were voted to Mr. Nelson for his com- 

 munication. 



The following letter from Dr. M. D. Ewell, in reply to the criticisms 

 made upon portions of his paper, " On the Present Status of Micrometry," 

 read at the November Meeting of the Society, was read by Mr. J. W. 

 Gordon : — 



The American Microscopical Society, 

 January 11th, 1909. 



My Dear Mr. Gordon, 



I am very much gratified that my paper excited so much criticism. 

 Irrespective of its merits, it has accomplished its purpose. I wish to 

 thank each of the gentlemen whose remarks appear in the Proceedings, 

 for their interest in the paper. 



Some of the criticisms are well taken, and some, it seems to me, are, 

 owing to misapprehension of my objects and methods, not well taken. 



The absolute values, as suggested by Mr. Conrady, are of the 

 greatest value to the user of the scale ; but it was not my object to 

 standardise these scales, but only to show their want of agreement with 

 each other. As a matter of fact, however, the results published will, I 

 think, be found not far out of the way if the total lengths are computed 

 from the micrometer values given in the paper. In 1882 the American 

 Microscopical Society procured a standard centimeter " Cm. A " on 

 platin-iridium divided to millimeters, the first millimeter to tenths, and 

 the first one-tenth to hundredths of a millimeter. This was standardised 

 by the United States Coast Survey (the Bureau of Weights and 

 Measures) and also by Prof. W. A. Kogers, and others. The stage and 

 filar micrometers used by me in this investigation were calibrated by 

 direct comparison with " Cm. A," the mean of many series of readings, 

 running into thousands, being used. As to the powers used, I am 

 satisfied from long experience that there is no advantage in the use of 

 high powers in metrology. See the results of several series of measure- 

 ments in 1885-1889, published in the Proceedings of the American 

 Society of Microscopists. 



Mr. Cheshire is in error in stating that no attempt had been made to 

 utilise the same part of the (filar) micrometer screw for the different 

 measurements. Perhaps the work is open to criticism in using different 

 filar micrometers, though I think not, as they were each and all very 

 carefully calibrated on a well determined unit of the same length as the 

 unit measured, always using the same part of the screw. I was very 



