So PRACTICAL HISTOLOGY. 



fying power, be equivalent to l0 ooo inch. This ascer- 

 tained, substitute the blood for the stage micrometer. 

 Suppose that the broad diameter of a coloured corpuscle 

 covers three divisions of the ocular micrometer (Fig. 40), 

 then I0 g 00 = ^VF inch. The corpuscles vary slightly 

 in size ; the average size being ^Q inch. 



6 1. It is very desirable that a uniform system of 

 measurement should be adopted by histologists. We em- 

 ploy the metric system for fluid measures and weights, and 

 it seems very desirable also to use it for measures of length. 

 The expression of lengths in fractions of an inch is always 

 clumsy, and apt to lead to confusion. The micromillimeter 

 is the best histological unit ; and it is very conveniently 

 written by the Greek /a, and in speaking, shortened to 

 micro. A micromillimeter is the loooth of a millimeter 

 (0*0000397 inch). 



In using the millimeter instead of the English stage 

 micrometer for the above observation, its divisions (o'oiMM) 

 will be seen through the lines of the ocular micrometer. 

 Probably, with the magnifying power employed (300 diam.), 

 it will be found that, with the tube somewhat shortened, 

 three ocular micrometer divisions cover one division of the 

 stage micrometer. If 3 oc. mic. divisions = o'oiMM; 

 evidently i oc. mic. division must = 0^003 3 millimeter; that 

 is, 3 '3 micromillimeter s. With the tube of the microscope 

 thus shortened, a red blood corpuscle would be found to 

 cover very nearly 2*5 oc. mic. divisions ; hence 3*3 x 2*5 = 

 8*25 /i. The average size is 8 i*. By remembering this 

 figure for the blood corpuscle, it is easy to have an idea of 

 the size of an object when expressed in micromillimeters. 



