PHILOSOPHICAL SOCIETY OF WASHINGTON. 23 



sandths of a millimetre as tlie unit, wliich is very convenient in 

 a large number of cases, the simplest way is to use a magnifying 

 power that will make ten divisions of the eyepiece-micrometer 

 exactly coincide with one-hundredth of a millimetre on the stage- 

 micrometer. The glass eyepiece-micrometer should have a scale 

 a centimetre long ruled in one hundred parts. By increasing the 

 power so that a larger number than ten of these divisions shall 

 correspond to one-hundredth of a millimetre on the stage-micro- 

 meter, a unit of any degree of minuteness that may be required 

 for any special work can be obtained up to the limits of distinct 

 vision with the microscope. 



B. But although I regard the hundredth of a millimetre as a 

 very eligible dimension for the closest divisions of the stage- 

 micrometer, when it comes to expressing the results of our mea- 

 surement in speech or writing, I do not think it is convenient to 

 use the hundredth of a millimetre as the unit of expression. It 

 is too large, and the results of too many measurements would 

 still have to be expressed in decimal fractions. The thousandth 

 of a millimetre is much more convenient as a unit of expression, 

 and I would advise that microscopists should agree to call this 

 dimension a micron, and represent it in writing by the Greek 

 letter ^. This dimension has already been adopted as the unit 

 of expression by a number of European microscopists, who rep- 

 resent it by the same Greek letter, but call it a micro-millimetre. 

 The term micron should, I think, be preferred because well known 

 to scientific men other than microscopists, having for some time 

 been used in expressing minute differences by those officially 

 engaged in 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 and 5. To obtain a suitable standard stage-micrometer, I 

 would advise each microsco] ical society to select one ruled as 

 above described, by any peison 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 eyepiece-microm- 

 eter, that the divisions agree among themselves. This is com- 

 paratively 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 microscopical society to send its stand- 



