128 MAGNIFICATION AND MICROMETRY [CH.1T 



'i 183. Micrometry by the use of a stage micrometer on which to mount 

 the object. 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. 



184. Micrometry by dividing the size of the image by the 

 magnification of the microscope. For example, employ the 3 mm. 

 ( l /% 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 

 ( 176). Divide the size of the image in each case by the magnifi- 

 cation, and the result gives 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 microscope 400 diameters ( 170), then 

 the actual length of this long axis of the corpuscle is 18 mm. -f- 400 

 =0.045 mm - or 45/^ ( 182). 



FIG. 116. Preparation of blood with 

 a ring around a group of blood cor- 

 puscles. 



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, marking on the sketch the corpuscles measured (Figs. 70, 

 75). The different corpuscles vary considerably in size, so that 

 accurate comparison of different methods of measurement can only 



ings 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 inter- 

 national 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. 



