116 SUMMARY OF CURRENT RESEARCHES RELATING TO 



preparing standard measures of length, and having been adopted by 

 the International Metric Commission. I think it running an 

 unnecessary risk of confusion to select any other than this well- 

 recognized term for the dimension in question. 



4 & 5, To obtain a suitable standard stage-micrometer, I would 

 advise each microscopical society to select one ruled, as above 

 described, by any person in whom they have confidence, and to satisfy 

 themselves by comparison of the several parts with each other, by 

 means of the same part of the eye-piece micrometer, that the divisions 

 agree among themselves. This is comparatively easily done ; the 

 real difficulty will be to determine whether the whole scale is really 

 a centimetre long. To ascertain this, I would advise each micro- 

 scopical society to send its standard micrometer to the Superintendent 

 of the Coast Survey at Washington, with the request that he will have 

 it compai-ed with a recognized standard in the Bureau of Weights and 

 Measures, and return it with a report of the error, if any. I have 

 reason to believe that such requests would be promptly and courteously 

 responded to. Each society should then preserve the standard thus 

 obtained for the sole purpose of enabling its members to compare 

 their stage-micrometers with it. I think this plan much wiser than 

 to relegate the question to any one of the ingenious men who are 

 endeavouring in this country, with considerable success, to make 

 accurate rulings on glass, and I should anticipate better results from 

 it than from the appointment of a special committee of the American 

 Society of Microscojjists to prepare a standard scale. 



In conclusion, I readily admit that so long as the English 

 microscopists continue to express the results of their measurements in 

 decimals of an English inch, there will be American microscopists 

 who will do the same, either for all purposes or for particular work, 

 and of course it is very desirable that these measurements also should 

 be accurate. The stage-micrometers on this system in the market are 

 usually ruled in hundredths and thousandths of an inch. The latter 

 divisions are too wide to give values to the eye-piece micrometer with 

 the higher powers, while the five-thousandths, ten-thousandths, or even 

 finer divisions, ruled also on some of these micrometers, are incon- 

 veniently close. I would advise the makers to rule such micrometers 

 four- tenths of an inch long, divided into hundredths of an inch, one of 

 the hundredths being subdivided into ten, another into twenty-five 

 spaces. These latter sjiaces, each representing one twenty-five- 

 hundredth of an inch, sufficiently approximate the hundredth of a 

 millimetre to be used with equal convenience with the higher powers. 

 The scale on the glass eye-piece micrometer, used with these stage- 

 micrometers, should be, if specially made for the purpose, four-tenths 

 of an inch long, divided into one hundred parts, each one two-hundred- 

 and-fiftieth of an inch ; but these divisions would so closly approximate 

 those of the metric eye-piece micrometer proposed, that it might be 

 used without inconvenience instead. Where it is thought worth 

 while by a microscopical society to procure a standard scale of this 

 kind, it should be sent to the Coast Survey Office for measurement, as 

 in the case of the metric scales." 



