4 1 MA GNIFICA TION AND DRA WING. 



MICROMETRY WITH THE SIMPLE MICROSCOPE. 



§ 107. 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 (§ 99). 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 di- 

 visions on the rule in getting the spread of the dividers. 



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

 determining the power (§ 98), measure the image at the standard dis- 

 tance. 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 varying excellence for obtaining the size 

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

 micrometer (§§ 116, 117) being most accurate. 



§ 108. 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, Hurting, in his work on the microscope (1859), proposed 

 the one thousandth of a millimeter ( 10 1 00 mm. or 0.001 mm.) or 

 one millionth of a meter ( 100 ^ FDir or o. 00000 1 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 micrnni. In English it is most often written Micron, 

 plural micro, or microns, pronunciation Mic'ron, or Micron. By uni- 

 versal 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 ( l0 5 00 or o.oo5ths mm.) thus, 5/*.* 



* The term Microniillimeter ab. mmm. is very cumbersome, and besides is en- 

 tirely inappropriate since the adoption of definite meanings for the prefixes micro 

 and mega, meaning respectively one millionth and one million times the unit be- 

 fore which it is placed. A microniillimeter would then mean one-millionth of a 

 millimeter, not one-thousandth. The term micron, has been adopted by the great 

 microscopical societies, the international commission on weights and measures and 

 by original investigators, and is in the opinion of the writer the best term to em- 

 ploy. Jour. Roy. Micr. Soc, 1888, p. 502 ; Nature, Vol. XXXVII, (188S), p. 388. 



