CH. IV] MAGNIFICATION AND MICROMETRY 127 



mine when the points of the dividers exactly include the object. 

 The spread of the dividers is then obtained as above (§ 173) . This 

 amount will be the actual size of the object, as the microscope was 

 only used in helping to see when the divider points exactly enclosed 

 the object, and then for reading the divisions on the rule in getting 

 the spread of the dividers. 



(B) One may put the object under the simple microscope and 

 then, as in determining the power (§ 172), measure the image at 

 the standard distance. If the size of the image so measured is 

 divided by the magnificatibn of the simple microscope, the quotient 

 give the actual size of the object. One might use the eikonometer 

 also (§ 196). 



Use a fly's wing or some other object of about that size, and 

 try to determine the width in the two ways described above. If all 

 the work is accurately done the results will agree. 



MICROMETRY WITH THE COMPOUND MICROSCPE 



There are several ways of varying excellence for obtaining the 

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

 ocular micrometer (§ 189-193) being most accurate. 



§ 182. 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 (t-jVt mm - 

 or 0.001 mm.) or one millionth of a meter (niwiinr or 0.00000 1 

 meter) as the unit. He named this unit micro-millimeter and 

 designated it mmm. In 1869, Listing (Carl's Repetorium fur Ex- 

 perimentai-Physik, Bd, X, p. 5) favored the thousandth of a milli- 

 meter as unit and introduced the name Mikron or micrum. In 

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

 pronunciation Mik'rSn or Mik'rbn). 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 

 (nnnr or °'°°5 mm -> thus > SM-* 



* The term micromillimeter, abbreviation mmm., is very cumbersome, 

 and besides is entirely inappropriate since the adoption of the definite mean- 



