112 MAGNIFICATION AND MICROMETRY [CH. IV 



made, and also the ways in which variations in magnification and the val- 

 uation of the ocular micrometer may be produced ( 161. 162, 172, 176). 



MICROMETRY 



164. Micrometry is the determination of the size of objects by 

 the aid of a microscope. 



MICROMETRY WITH THE SIMPLE MICROSCOPE 



165. With a simple microscope (A), the easiest and best way 

 is to use dividers and then with the simple microscope determine 

 when the points of the dividers exactly include the object. The spread 

 of the dividers is then obtained as above ( 157). 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 ( 156), measure the image at the 

 standard distance. If 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 varying excellence for obtaining the size 

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

 micrometer ( 175-176) being most accurate. 



166. Unit of Measure in Micrometry. As most of the ob- 

 jects 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^^ mm. or o.ooi mm.) or 

 one millionth of a meter (-nnriinnr or o.oooooi 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) 



