150 MEASURING WITH THE MICROSCOPE [Ch. V 



Micrometry with the -Compound Microscope 



There are several ways of varying excellence for obtaining the size 

 of objects with the compound microscope, the method with the ocular 

 micrometer (§ 238) being most accurate. 



§ 246. Unit of measure in micrometry. — As most of the objects 

 measured with the compound microscope are smaller than any of the 

 originally named divisions of the meter, and the common or decimal 

 fractions necessary to express the size are liable to be unnecessarily 

 cumbersome, Harting, in his work on the microscope (1859), proposed 

 the one-thousandth of a millimeter (0.001 mm.) or one-millionth of a 

 meter (0.000001 meter) as the unit. He named this unit micro- 

 millimeter and designated it mmm. In 1869, Listing (Carl's Reper- 

 torium fiir Experimental-Physik, Bd. X, p. 5) favored the thousandth 

 of a millimeter as unit and introduced the name mikron or micrum. 

 In English it is most often written Micron (plural micra or microns, 

 pronunciation Mik'ron or Mik'ron). By universal consent the sign 

 or abbreviation used to designate it is the Greek [X. Adopting this 

 unit and sign, one would express five-thousandths of a millimeter 

 (0.005 mm.) thus, 5 /a. 



§ 216a. The term "micromillimeter," abbreviation mmm., is very cumber- 

 some, and besides is entirely inappropriate, since the adoption of the definite 

 meanings for the prefixes micro and mega, meaning respectively one-millionth 

 and one million times the unit before which it is placed. A micromillimeter 

 would then mean one-millionth of a millimeter, not one-thousandth. The 

 term " micron " has been adopted by the great microscopical societies, the inter- 

 national commission on weights and measures, and by original investigators, 

 and is, in the opinion of the writer, the best term to emplov. Jour. Roy. 

 Micr. Soc, 1888, p. 502; Nature, Vol. XXXVII (1888), p. 388.' 



§ 247. Micrometry by the use of a stage micrometer on which to 

 mount the object. — In this method the object is mounted on a mi- 

 crometer and then put under the microscope, and the number of 

 spaces covered by the object is read off directly. It is exactly like 

 putting any large object on a rule and seeing how many spaces of the 

 rule it covers. The defect in the method is that it is impossible to 

 properly arrange objects on the micrometer. Unless the objects 

 are circular in outline they are liable to be oblique in position, and in 

 every case the end or edges of the object may be in the middle of a 



