ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 115 



3. What unit, within the system selected, is most eligible ? 



4. What steps should be taken to obtain a suitable standard 

 measure of this unit ? 



5. How can this standard micrometer be best preserved and made 

 useful to all parties concerned ? " 



The reply was as follows : — 

 "1. I am in favour of the adoption of a suitable standard for 

 micrometry by the American Society of Microscopists at their next 

 meeting. 



2. For this particular purpose I think the metric system offers so 

 many conveniences that I favour its employment. 



3. The selection of an eligible unit within the system involves, it 

 appears to me, two distinct questions: A. How shall the stage- 

 micrometer be ruled ? B. How shall the measurements made, be 

 expressed in speech or writing ? 



A. The object of the stage-micrometer is chiefly to give values to 

 the divisions of the eye-piece micrometer with the power used in any 

 given case. It should be long enough to be used for this purpose with 

 the lowest powers of the compound Microscope, and have a part of its 

 length ruled sufficiently close to answer the same end with the highest 

 powers. I favour the adoption of a standard scale a centimetre long 

 ruled in millimetres, and one of these ruled in hundredths. I have 

 used stage-micrometers ruled in thousandths of a millimetre, but 

 regard such divisions as inconveniently close for this purpose. To 

 measure in thousandths of a millimetre as the unit, which 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 eye-piece micro- 

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

 stage-micrometer. The glass eye-piece 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-micrometer, 

 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 measurement 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 /x. This dimension has already been adopted as the 

 unit of expression by a number of European microscopists, who 

 represent 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 



I 2 



