1 5 MEASURING WITH THE MICROSCOPE [Cn. 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. Most of the objects measured with 

 the compound microscope, and many of those in physics and chemistry are smaller, 

 often much smaller, than any of the originally named divisions of the meter. 

 To express these very small dimensions in common or in decimal fractions of a 

 meter or millimeter is not only cumbersome, but liable to give rise to errors, con- 

 sequently workers in microscopy, in physics and in chemistry have sought to avoid 

 the difficulties by selecting and naming as units such small divisions of the meter 

 that the minute dimensions can be expressed as whole numbers. 



Up to the present three such special units have been designated and have re- 

 ceived the sanction and use of the highest authorities. They are: 



1. The Micron (symbol /*). This is the one millionth of a meter (o.ooo ooi, m.); 

 one thousandth of a millimeter (o.ooi mm.); ten thousand Angstrom units (ic,ooo 

 A.U.); one thousand millimicrons (1000 mju). 



2. The Angstrom Unit (A.U.) or Tenthmeter (io~ 10 m.). It is the one ten 

 billionth of a meter (o.ooo ooo ooo i m) ; the ten thousandth of a micron (o.ooo i /i) ; 

 the one tenth of a millimicron (o.i m/x). 



3. The Millimicron (mju). This is the one billionth of a meter (o.ooo ooo ooi m.) ; 

 the one thousandth of a micron (o.ooi ju); ten Angstrom units (10 A.U.). 



The Micron unit (ju) has been generally adopted in microscopy, and is widely 

 used for minute sizes in all branches of science. Harting recommended it for mi- 

 croscopy in 1859, but he named it micro-millimeter, or milli-millimeter, and gave 

 as a symbol mmm. Since the definite meaning for micro, as one millionth of the 

 unit before which it is placed, has been decided on by metrologists, micro-milli- 

 meter should mean one millionth of a millimeter, not one thousandth. Harting' s 

 Milli-millimeter is correct, but awkward. Occasionally one meets the symbol jiiju 

 for millimicron (m/z). /z/x should stand for the millionth, not for the thousandth, 

 of a micron. 



See Jour. Roy. Micr. Jour. Soc., 1888, p. 502 Nature, Vol. XXXVII, p. 388; 

 Bit. Bureau Standards, Vol. VIII, p. 540. 



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 



