CH. IV.} MAGNIFICATION AND MICROMETRY. 101 



ing this unit and sign, one would express five thousandths of a milli- 

 meter (yrnnf or o.oosths mm.) thus, 5/x.* 



I 15S. 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 

 msy 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. 



§ 159. Micrometry by dividing the size of the image by the magjiifi- 

 cation of the microscope. — For example, employ the 3 mm. (}i in.) ob- 

 jective, 25 mm. (1 in.) ocular, and a Necturus' red blood-corpuscle prepa- 

 ration 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 ob- 

 taining the magnification of the microscope (§ 151). 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 (§ 145), then the actual length of this long axis of 

 the corpuscle is 18 mm. -5- 400 = .045 mm. or 45 p (§ 157). 



Fig. 97. 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, marking on 

 the sketch the corpuscles measured. The different corpuscles vary con- 

 siderably in size, so that accurate comparison of different methods of 

 measurement can only be made when the same corpuscles are measured 

 in each of the ways (Figs. 61-66). 



*The term Micromillimeter ab. mmui. 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 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. 



