CH. IV] MAGN1FICA TION AND MICROMETR Y 113 



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 Mik'r6n or Mik'r6n). By universal con- 

 sent the sign or abbreviation used to designate it is the Greek //. 

 Adopting this unit and sign, one would express five thousandths of a 

 millimeter ( T -<jVo or -5 mm.) thus, 5//.* 



\ 167. Micrometry by the use of a stage micrometer on which to mount the ob- 

 ject. In this method the object is mounted on a micrometer 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 space instead of against one of the lines, consequently 

 the size must be estimated or guessed at rather than really measured. 



1 68 . Micrometry by dividing the size of the image by the magnifica- 

 tion of the microscope. For example, employ the 3 mm. (^6 in.) objective, 

 25 mm. (i in.) ocular, and a Necturus' red blood-corpuscle preparation 

 as object. Obtain the size of the image of the long and short axes 

 of three corpuscles with the camera lucida and dividers, exactly as in 

 obtaining the magnification of the microscope ( 160). Divide the size 

 of the image in each case by the magnification, and the result will be 

 the actual size of the blood-corpuscles. Thus, suppose the image of the 

 long axis of the corpuscle is 18 mm. and the magnification of the micro- 

 scope 400 diameters ( 154), then the actual length of this long axis of 

 the corpuscle is 18 mm.-r- 400=0.045 mm. or 45^ ( 166). 



FIG. 103. Preparation of blood with a 

 ring around a group of blood corpuscles. 



As the same three blood-corpuscles are to be measured in three 

 ways, it is an advantage to put a delicate ring around a group of three 

 or more corpuscles, and make a sketch of the whole enclosed group, 



*The term micromillimeter, abbreviation mmm., is very cumbersome, and be- 

 sides is entirely inappropriate since the adoption of the definite meanings for the 

 prefixes micro and mega, meaning respectively one-millionth and one million 

 times the unit before which it is placed. A micromillimeter 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 employ. Jour. Roy. Micr. Soc., 1888, p. 502 ; Nature, 

 Vol. XXXVII (1888), p. 388. 



