HISTOLOGICAL DEMONSTRATIONS. 63 



The diluted blood is now introduced into a capillary glass tube (Fig. 

 42) flattened and fixed to a glass slide. Near the upper surface of this 



FIG. 42. Capillary tube for enumeration of blood corpuscles. (After Malassez.} 



tube there is an exceedingly fine elliptical cavity, the capacity of 

 various lengths of which is carefully ascertained by the maker. The 

 different lengths are tabulated in micromillimeters ( 61) in the left 

 column of numbers engraved on the slide (Fig. 42), while the capacities 

 of these various lengths are expressed in fractions of a cubic millimeter 

 in the right column ; e.g. 500 p contain yl^ of a cubic millimeter. 



The capillary is filled with the diluted blood by placing a drop at the 

 extremity, and allowing it to enter by capillary attraction. It may be 

 necessary, however, to hasten its entrance by aspiration. As soon as 

 the glass tube is filled, the remainder of the drop must be carefully 

 removed by bibulous paper, otherwise the corpuscles will not come to 

 rest within the tube. 



Suppose we desire to count the corpuscles in 500 micromillimeters 

 of the capillary, we proceed thus : A millimeter stage micrometer 

 is focalised by a power of about 100 diameters. A squared micro- 

 meter (Fig. 43) is placed in the eye-piece, and the tube of the 

 microscope is shortened or lengthened until the whole breadth viz. 

 ten squares of the ocular micrometer exactly include 500 micromilli- 

 meters of the stage micrometer. It avoids future trouble if a circular 

 line be drawn on the tube of the microscope to indicate its position 

 when the ten squares of the ocular micrometer have this value, with a 

 given combination of lenses. 



The capillary tube with the blood is now substituted for the stage 

 micrometer, care being taken not to alter the length of the tube of the 

 microscope. Fig. 43 represents the capillary seen through the squared 

 micrometer. The coloured corpuscles within the whole of the 500 

 micromillimeters of the capillary have now to be counted. The squares 

 in the eye-piece facilitate the process, and the number found in each 

 square should be tabulated to avoid error. 



Suppose that 334 coloured corpuscles are counted in 500 micro- 

 millimeters of the tube, this would, in a tube of the calibre of that 

 under consideration, be the number in j^j part of a cubic millimeter 



