232 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



the slit being so adjusted by means of the little knob that four squares 

 of the central platform of the counting chamber just coincide with it. 

 The number of red corpuscles are counted, and the preparation may now 

 be shifted as many times as desired, each count representing the number 

 of corpuscles in four squares, since the slit corresponds exactly to four of 

 the squares. The total number obtained after several such counts being 

 divided by the number of counts, gives the number of red corpuscles per 



Fig. 27. 



field of four squares ; hence division by four gives the number per square. 

 This number multiplied by 4000 would represent the number of corpuscles 

 per cubic millimetre were it not that the dilution has to be taken into 

 account, and accordingly the result must be multiplied by 100 or 200. 



Example : Average number of red corpuscles per square = 10. Then 

 10 x 4000 x 100 = 4,000,000 per cubic millimetre. 



The method is quicker and more accurate than that usually employed, 



Fig. 28. 



since it enables a much larger number of squares to be counted irrespec- 

 tive of the lines of the counting chamber, which constitute an element 

 of confusion in the process of counting. 



Again, the Blenden ocular may be used for the purpose of counting 

 the leucocytes in the following manner. A dry-film preparation of the 

 blood to be examined is fixed and stained by Irishman's or Jenner's stain. 

 Using a No. 9 Leitz objective and a Blenden ocular, the number of red 



