I oo MA GNIFICA TION AND MICRO ME TR Y. [CH. I V. 



MICROMETRY. 



§ 155. Micrometry is the determination of the size of objects by the 

 aid of a microscope. 



MICROMETRY WITH THE SIMPLE MICROSCOPE. 



§ 156. With a simple microscope (A), the easiest and best way is to 

 use dividers and then the simple microscope to see when the points of 

 the dividers exactly include the object. The spread of the dividers is 

 then obtained as above (§ 148). 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 (§ 147), measure the image at the standard 

 distance. If now the size of the image so measured is divided by the 

 magnification of the simple microscope, the quotient will give the actual 

 size of the object. 



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 MICROSCOPE. 



There are several ways of va^ing excellence for obtaining the size 

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

 micrometer (§ 166-167) being most accurate. 



§ 157. 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 (y^Tr mm. or 0.001 mm.) or 

 one millionth of a meter (urowo^ or °- 000001 meter) as the unit. He 

 named this unit micro-millimeter and designated it mmm. In 1869, 

 Listing (Carl's Repetorium fur 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 Mic'rSn or Mic'rSu. By universal con- 

 sent the sign or abbreviation used to designate it is the Greek fx. Adopt- 



