282 



BLOOD AND BLOOD EXAMINATION 



A drop of the diluted solution is allowed to fall upon the counting cham- 

 ber, the cover-glass is placed over it, and this is permitted to sediment for a 

 short time. [Tiirck's emendation of this step in technic is of value. Before 



O.lOOimti. 



C.Zeiss 

 I Jena, 



Fig. 14. — Hemocytometer. (After Thoma.) 



blowing out a drop of diluted blood upon the counting-disc, he puts on 2 

 corners of the shelf (IF, Fig. 14), that is, to support the cover-glass, a minute 

 drop of water. The diluted blood drop is then adjusted on the counting disc 

 and as rapidly as possible the cover-glass is let down and pressed firmly into 



position by strong pressure with the thumbs. 

 Newton's rings are then visible at once, and the 

 cover-glass is sealed firmly in position. — Ed.] 

 After which (using about 300 diameters magnifi- 

 cation) as many as possible of the squares in view 

 should be counted. If, for example, we have 

 diluted the blood 1 : 100 and then count in 33 

 squares (Fig. 15) 300 red blood-corpuscles, the 

 following calculation is made (we must multiply 

 with 100 to compensate for the blood dilution) : 



r^ TT "^^T~r^~~^ TT ^"r* 



300 X 4000 X 100 

 32 



= 3 : 750,000 ; 



Fig. 15. — The Microscopic 

 Picture with Blood-Cor- 



pxrscLEs. (After Landois.) This is the number of red blood-corpuscles con- 

 tained in a c.mm. 



Any one in constant practice with this instrument possesses an exact meas- 

 uring apparatus in which the danger of error is very slight; naturally, the 

 aggregate of inaccuracies is less when more squares are counted. [Of course 

 the amount of error depends on the number of squares counted. With men 

 properly trained no constant practice is needed. My own practice is to count 

 100 squares. The error is then negligible. — Ed.] 



It must be noted that, particularly in extreme anemia, the number of 

 blood-corpuscles may easily be reckoned as too low, since the mierocytes which 

 occur here are apt to be missed if we are using a low power. Obviously the 

 observations we have made in regard to the variation in the composition of 



